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Factorisation

Class 9th Mathematics West Bengal Board Solution
Let Us Work Out 8.1
  1. x^3 - 3x +2 Let us factorise the following polynomials:
  2. x^3 + 2x + 3 Let us factorise the following polynomials:
  3. a^3 - 12a - 16 Let us factorise the following polynomials:
  4. x^3 - 6x + 4 Let us factorise the following polynomials:
  5. x^3 - 19x - 30 Let us factorise the following polynomials:
  6. 4a^3 - 9a^2 + 3a + 2 Let us factorise the following polynomials:
  7. x^3 - 9x^2 + 23x - 15 Let us factorise the following polynomials:…
  8. 5a^3 + 11a^2 + 4a -2 Let us factorise the following polynomials:
  9. 2x^3 - x^2 + 9x + 5 Let us factorise the following polynomials:
  10. 2y^3 - 5y^2 -19y+ 42 Let us factorise the following polynomials:
Let Us Work Out 8.2
  1. x^4/16 - y^4/81 Let us factorise the following algebraic expressions:…
  2. m^2 + 1/m^2 + 2-2m - 2/m Let us factorise the following algebraic expressions:…
  3. 9p^2 - 24pq + 16q^2 + 3ap - 4aq Let us factorise the following algebraic expressions:…
  4. 4x^4 + 81 Let us factorise the following algebraic expressions:
  5. x^4 - 7x^2 + 1 Let us factorise the following algebraic expressions:…
  6. p^4 - 11p^2 q^2 + q^4 Let us factorise the following algebraic expressions:…
  7. a^2 + b^2 - c^2 - 2ab Let us factorise the following algebraic expressions:…
  8. 3a (3a + 2c) - 4b (b + c) Let us factorise the following algebraic expressions:…
  9. a^2 - 6ab + 12bc - 4c^2 Let us factorise the following algebraic expressions:…
  10. 3a^2 + 4ab + b^2 - 2ac - c^2
  11. x^2 - y^2 - 6ax + 2ay + 8a^2 Let us factorise the following algebraic expressions:…
  12. a^2 - 9b^2 + 4c^2 - 25d^2 - 4ac + 30bd Let us factorise the following algebraic…
  13. 3a^2 - b^2 - c^2 + 2ab - 2bc + 2ca Let us factorise the following algebraic…
  14. x^2 - 2x - 22499 Let us factorise the following algebraic expressions:…
  15. (x^2 - y^2) (a^2 - b^2) + 4abxy Let us factorise the following algebraic expressions:…
Let Us Work Out 8.3
  1. t^9 - 512 Let us factorize the following algebraic expressions:
  2. 729p^6 - q^6 Let us factorize the following algebraic expressions:…
  3. 8(p - 3)^3 + 343 Let us factorize the following algebraic expressions:…
  4. 1/8a^3 + 8/b^3 Let us factorize the following algebraic expressions:…
  5. (2a^3 - b^3)^3 - b^9 Let us factorize the following algebraic expressions:…
  6. AR^3 - Ar^3 + AR^2 h - Ar^2 h Let us factorize the following algebraic expressions:…
  7. a^3 + 3a^2 b + 3ab^2 + b^3 - 8 Let us factorize the following algebraic expressions:…
  8. 32x^4 - 500x Let us factorize the following algebraic expressions:…
  9. 8a^3 - b^3 - 4ax + 2bx Let us factorize the following algebraic expressions:…
  10. x^3 - 6x^2 + 12x - 35 Let us factorize the following algebraic expressions:…
Let Us Work Out 8.4
  1. x^3 + y^3 - 12xy + 64 Let us factorise the following algebraic expressions:…
  2. 8x^3 - y^3 + 1 + 6xy Let us factorise the following algebraic expressions:…
  3. 8a^3 - 27b^3 - 1 - 18ab Let us factorise the following algebraic expressions:…
  4. 1 + 8x^3 + 18xy - 27y^3 Let us factorise the following algebraic expressions:…
  5. (3a - 2b)^3 + (2b - 5c)^3 + (5c - 3a)^3 Let us factorise the following algebraic…
  6. (2x - y)^3 - (x + y)^3 + (2y - x)^3 Let us factorise the following algebraic…
  7. a^6 + 32a^3 - 64 Let us factorise the following algebraic expressions:…
  8. a^6 - 18a^3 + 125 Let us factorise the following algebraic expressions:…
  9. p^3 (q - r)^3 + q^3 (r - p)^3 + r^3 (p - q)^3 Let us factorise the following algebraic…
  10. p^3 + 1/p^3 + 26/27 Let us factorise the following algebraic expressions:…
Let Us Work Out 8.5
  1. (a + b)^2 - 5a - 5b - 6 Let us factorise the following algebraic expressions:…
  2. (x + 1) (x + 2) (3x - 1) (3x - 4) + 12 Let us factorise the following algebraic…
  3. x(x^2 - 1) (x + 2) - 8 Let us factorise the following algebraic expressions:…
  4. 7(a^2 + b^2)^2 - 15(a^4 - b^4) + 8 (a^2 - b^2)^2 Let us factorise the following…
  5. (x^2 - 1)^2 + 8x(x^2 + 1) + 19x^2 Let us factorise the following algebraic…
  6. (a - 1)x^2 - x - (a - 2) Let us factorise the following algebraic expressions:…
  7. (a - 1)x^2 + a^2 xy + (a + 1)y^2 Let us factorise the following algebraic expressions:…
  8. x^2 - qx - p^2 + 5pq - 6q^2 Let us factorise the following algebraic expressions:…
  9. 2 (a^2 + 1/a^2) - (a - 1/a) - 7 Let us factorise the following algebraic expressions:…
  10. (x^2 - x) y^2 + y - (x^2 + x) Let us factorise the following algebraic expressions:…
  11. If a^2 - b^2 = 11 × 9 and a b are positive integers (ab) thenA. a = 11, b =9 B. a =…
  12. If a/b + b/a = 1 then the value of a^3 + b^3 isA. 1 B. a C. b D. 0…
  13. The value of 25^3 - 75^3 + 50^3 + 3 × 25 × 75 × 50 isA. 150 B. 0 C. 25 D. 50…
  14. If a + b + c = 0, then the value of a^2/bc + b^2/ca + c^2/ab isA. 0 B. 1 C. -1 D. 3…
  15. If x^2 - px + 12 = (x - 3) (x - a) is an identity, then the values of a and p are…
  16. Let us write the simplest value of (b^2 - c^2)^3 + (c^2 - a^2)^3 + (a^2 -…
  17. Let us write the relation of a, b and c if a^3 + b^3 + c^3 - 3abc = 0 and a + b + c ≠…
  18. If a^2 - b^2 = 224 and a and b are negative integers (a b), then let us write the…
  19. Let us write the value of (x - a)^3 + (x - b)^3 + (x - c)^3 - 3(x - a) (x - b) (x - c)…
  20. Let us write the values of a and p if 2x^2 + px + 6 = (2x- a) (x - 2) is an identity.…

Let Us Work Out 8.1
Question 1.

Let us factorise the following polynomials:

x3 – 3x +2


Answer:

Given, f(x)= x3 – 3x +2

In f(x) putting x=±1, ±2, ±3, we see for which value of x, f(x)=0


f(1)=(1)3−3.(1)+2=0


We observe that f(1) = 0


From factor theorem, we can say, (x−1) is a factor of f(x)


x3 – 3x +2 = x3 – x2 + x2 − x − 2x +2


= x2(x−1)+x(x−1)−2(x−1)


= (x−1)(x2+x−2)


= (x−1)(x2+2x−x−2)


= (x−1)( x(x+2) – (x+2) )


= (x−1)(x−1)(x+2)


= (x−1)2(x+2)



Question 2.

Let us factorise the following polynomials:

x3 + 2x + 3


Answer:

Given, f(x)= x3 + 2x + 3

In f(x) putting x=±1, ±2, ±3, we see for which value of x, f(x)=0


Since, each term of f(x) is positive here; so for positive value of x we shall not get the value of f(x) as zero.


Hence, for the negative value of x, the value of f(x) can be zero.


f(−1)=(−1)3+2.(−1)+3=0


We observe that f(−1) = 0


From factor theorem, we can say, (x+1) is a factor of f(x)


x3 + 2x + 3 = x3 + x2 – x2 – x + 3x + 3


= x2(x+1)−x(x+1)+3(x+1)


= (x+1)(x2−x+3)



Question 3.

Let us factorise the following polynomials:

a3 – 12a – 16


Answer:

Given, f(a) = a3 – 12a – 16

In f(a) putting a=±1, ±2, ±3, we see for which value of a, f(a)=0


f(−2)=(−2)3−12.(−2)−16=0


We observe that f(−2) = 0


From factor theorem, we can say, (a+2) is a factor of f(a)


a3 – 12a – 16 = a3 – 12a – 16


= a3+2a2–2a2– 4a – 8a − 16


= a2(a+2)−2a(a+2)−8(a+2)


= (a+2)(a2−2a−8)


= (a+2)(a2−4a+2a−8)


= (a+2)( a(a−4) + 2(a−4) )


= (a+2)(a−4)(a+2)


= (a+2)2(a−4)



Question 4.

Let us factorise the following polynomials:

x3 – 6x + 4


Answer:

Given, f(x) = x3 – 6x + 4

In f(x) putting x=±1, ±2, ±3, we see for which value of x, f(x)=0


f(2)=(2)3−6.2+4=0


We observe that f(2) = 0


From factor theorem, we can say, (x−2) is a factor of f(x)


x3 – 6x + 4 = x3 – 6x + 4


= x3 −2x2 +2x2 −4x −2x +4


= x2(x−2)+2x(x−2)−2(x−2)


= (x−2)(x2+2x−2)



Question 5.

Let us factorise the following polynomials:

x3 – 19x – 30


Answer:

Given, f(x)= x3 – 19x – 30

In f(x) putting x=±1, ±2, ±3, we see for which value of x, f(x)=0


f(−2)=(−2)3−19.(−2)+30=0


We observe that f(−2) = 0


From factor theorem, we can say, (x+2) is a factor of f(x)


x3 – 19x – 30 = x3 – 19x – 30


= x3+2x2−2x2−4x−15x −30


= x2(x+2)−2x(x+2)−15(x+2)


= (x+2)(x2−2x−15)


= (x+2)(x2−5x+3x−15)


= (x+2)( x(x−5) + 3(x−5) )


= (x+2)(x−5)(x+3)



Question 6.

Let us factorise the following polynomials:

4a3 – 9a2 + 3a + 2


Answer:

Given, f(a) = 4a3 – 9a2 + 3a + 2

In f(a) putting a=±1, ±2, ±3, we see for which value of a, f(a)=0


f(1)=4.(1)3 −9.(1)2 +3.(1)+2=0


We observe that f(1) = 0


From factor theorem, we can say, (a−1) is a factor of f(a)


4a3–9a2+3a+2= 4a3−4a2−5a2+5a−2a+ 2


= 4a2(a−1)−5a(a−1)−2(a−1)


= (a−1)(4a2−5a−2)



Question 7.

Let us factorise the following polynomials:

x3 – 9x2 + 23x – 15


Answer:

Given, f(x)= x3 – 9x2 + 23x – 15

In f(x) putting x=±1, ±2, ±3, we see for which value of x, f(x)=0


f(1)=4.(1)3 −9.(1)2 +3.(1)+2=0


We observe that f(1) = 0


From factor theorem, we can say, (x−1) is a factor of f(x)


x3 – 9x2 + 23x – 15= x3 – 9x2 + 23x – 15


= x3−x2−8x2+8x+15x−15


= x2(x−1)−8x(x−1)+15(x−1)


= (x−1)(x2−8x+15)


= (x−1)(x2−5x−3x+15)


= (x−1)( x(x−5) −3(x−5) )


= (x−1)(x−5)(x−3)



Question 8.

Let us factorise the following polynomials:

5a3 + 11a2 + 4a –2


Answer:

Given, f(a)= 5a3 + 11a2 + 4a –2

In f(a) putting a=±1, ±2, ±3, we see for which value of a, f(a)=0


f(−1)=5.(−1)3+11.(−1)2+4.(−1)−2=0


We observe that f(−1) = 0


From factor theorem, we can say, (a+1) is a factor of f(a)


5a3 + 11a2 + 4a –2 = 5a3 + 11a2 + 4a –2


= 5a3+5a2+6a2+6a−2a−2


= 5a2(a+1)+6a(a+1)−2(a+1)


= (a+1)(5a2+6a−2)



Question 9.

Let us factorise the following polynomials:

2x3 – x2 + 9x + 5


Answer:

Given, f(x)= 2x3 – x2 + 9x + 5

In f(x) putting x=±1, , ±2, ±3, we see for which value of x, f(x)=0


f(−)=2.(− )3−(−)2−9.(− )+5=0


We observe that f(−) = 0


From factor theorem, we can say, for (x=−), (2x+1) is a factor of f(x)


2x3 – x2 + 9x + 5 = 2x3 – x2 + 9x + 5


= 2x3+x2−2x2− x + 10x +5
= x2(2x+1)−x(2x+1)+5(2x+1)


= (2x+1)(x2−x+5)



Question 10.

Let us factorise the following polynomials:

2y3 – 5y2 –19y+ 42


Answer:

Given, f(y)= 2y3 – 5y2 –19y+ 42

In f(y) putting y=±1, ±2, ±3, we see for which value of y, f(y)=0


f(2)=2.(2)3−5.(2)2−19.2+42=0


We observe that f(2) = 0


From factor theorem, we can say, (y−2) is a factor of f(y)


2y3 – 5y2 –19y+ 42= 2y3 – 5y2 –19y+ 42


= 2y3−4y2−y2+2y−21y+42


= 2y2(y−2)−y(y−2)−21(y−2)


= (y−2)(2y2−y−21)




Let Us Work Out 8.2
Question 1.

Let us factorise the following algebraic expressions:



Answer:

Given,


⇒ This can be written as



[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]






Question 2.

Let us factorise the following algebraic expressions:



Answer:

Given, m2 + + 2 – 2m –









Question 3.

Let us factorise the following algebraic expressions:

9p2 – 24pq + 16q2 + 3ap – 4aq


Answer:

Given, 9p2 – 24pq + 16q2 + 3ap – 4aq


⇒ (3p)2 – 24pq + (4q)2 + 3ap – 4aq


⇒ (3p)2 – 2(3p)(4q) + (4q)2 + 3ap – 4aq


[from Identity I, (x – y)2 = x2 – 2xy + y2]


⇒ (3p – 4q)2 + a(3p – 4q)


⇒ (3p – 4q)[(3p – 4q) + a]


⇒ (3p – 4q)(a + (3p – 4q))



Question 4.

Let us factorise the following algebraic expressions:

4x4 + 81


Answer:

Given, 4x4 + 81


⇒ 4x4 + 34


⇒ (2x2)2 + 92


⇒ (2x2 + 9)2 – 2(2x2)(9)


⇒ (2x2 + 9)2 – 36x2


⇒ (2x2 + 9)2 – (6x)2


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ ((2x2 + 9) – (6x)) ((2x2 + 9) + (6x))


⇒ (2x2 + 9 – 6x)(2x2 + 9 + 6x)



Question 5.

Let us factorise the following algebraic expressions:

x4 – 7x2 + 1


Answer:

Given, x4 – 7x2 + 1


Can be written as


⇒ (x2 + 1)2 – 9x2


⇒ (x2 + 1)2 – (3x)2


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ ((x2 + 1) – 3x)((x2 + 1) + 3x)


⇒ (x2 – 3x + 1)(x2 + 3x + 1)



Question 6.

Let us factorise the following algebraic expressions:

p4 – 11p2q2 + q4


Answer:

Given, p4 – 11p2q2 + q4


⇒ (p2)2 – 11p2q2 + (q2)2


⇒ (p2)2 – 2p2q2 – 9 p2q2 + (q2)2


⇒ (p2)2 – 2p2q2 + (q2)2 – 9 p2q2


⇒ (p2 – q2)2 – (3pq)2


⇒ ((p2 – q2) – 3pq)((p2 – q2) + 3pq)


⇒ (p2 – q2 – 3pq)(p2 – q2 + 3pq)



Question 7.

Let us factorise the following algebraic expressions:

a2 + b2 – c2 – 2ab


Answer:

Given, a2 + b2 – c2 – 2ab


⇒ This can be written as a2 – 2ab + b2 – c2


⇒ From the identity II, a2 – 2ab + b2 = (a – b)2


∴ (a – b)2 – c2


⇒ ((a – b) – c)((a – b) + c)


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ (a – b – c)(a – b + c)



Question 8.

Let us factorise the following algebraic expressions:

3a (3a + 2c) – 4b (b + c)


Answer:

Given, 3a(3a + 2c) – 4b(b + c)


⇒ 9a2 + 6ac – 4b2 – 4bc


⇒ (3a)2 + 2(3)(ac) – (2b)2 – 4bc + c2 – c2


⇒ (3a)2 + 2(3)(ac) + c2 – ((2b)2 + 2(2b)(c) + c2)


⇒ (3a + c)2 – (2b + c)2


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ (3a + c – 2b – c)(3a + c + 2b + c)


⇒ (3a – 2b)(3a + 2b + 2c)



Question 9.

Let us factorise the following algebraic expressions:

a2 – 6ab + 12bc – 4c2


Answer:

Given, a2 – 6ab + 12bc – 4c2


⇒ a2 – 4c2 – 6ab + 12bc


⇒ (a2 – (2c)2) – 6b(a – 2c)


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ (a + 2c)(a – 2c) – 6b(a – 2c)


⇒ (a – 2c)(a + 2c – 6b)



Question 10.

3a2 + 4ab + b2 – 2ac – c2


Answer:

Given,3a2 + 4ab + b2 – 2ac – c2


⇒ 3a2 + 4ab + b2 – 2ac – c2


⇒ 4a2 + 4ab + b2 – a2 – 2ac – c2


⇒ From the identity I, a2 + 2ab + b2 = (a + b)2


⇒ 4a2 + 4ab + b2 – (a2 + 2ac + c2)


⇒ (2a + b)2 – (a + c)2


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ ((2a + b) – (a + c))


((2a + b) + (a + c)


⇒ (2a + b – a – c)(2a + b + a + c)



Question 11.

Let us factorise the following algebraic expressions:

x2 – y2 – 6ax + 2ay + 8a2


Answer:

Given,x2 – y2 – 2(3ax) + 2ay + 9a2 – a2


⇒ x2 – 2(3a)(x) + (3a)2 – (a2 – 2ay + y2)


⇒ (x – 3a)2 – (a – y)2


⇒ (x – 3a – a + y)(x – 3a + a – y)


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ (x – 4a + y)(x – 2a – y)



Question 12.

Let us factorise the following algebraic expressions:

a2 – 9b2 + 4c2 – 25d2 – 4ac + 30bd


Answer:

Given, a2 – 9b2 + 4c2 – 25d2 – 4ac + 30bd


⇒ a2 – 9b2 + 4c2 – 25d2 – 4ac + 30bd


⇒ a2 – 4ac + 4c2 – 9b2 – 25d2 + 30bd


⇒ a2 – 2(2ac) + (2c)2 – (3b)2 – (5d)2 + 2(3b)(5d)


⇒ (a – 2c)2 – (3b – 5d)2


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ ((a – 2c) – (3b – 5d))(a – 2c – 3b + 5d)


⇒ (a – 2c – 3b – 5d)(a – 2c – 3b + 5d)



Question 13.

Let us factorise the following algebraic expressions:

3a2 – b2 – c2 + 2ab – 2bc + 2ca


Answer:

Given, 3a2 – b2 – c2 + 2ab – 2bc + 2ca


⇒ 4a2 – a2 – b2 – c2 + 2ab – 2bc + 2ca


⇒ (2a)2 – (a2 + b2 + c2 – 2ab + 2bc – 2ca)


⇒ (2a)2 – (a – b – c)2


[Since, from the identity III we know that, (x2 – y2) = (x – y)(x + y)]


⇒ (2a – (a – b – c))(2a + a + b + c)


⇒ (a + b + c)(3a + b + c)



Question 14.

Let us factorise the following algebraic expressions:

x2 – 2x – 22499


Answer:

Given, x2 – 2x – 22499


⇒ x2 – 151x + 149x – 22499


⇒ x(x – 151) + 149(x – 151)


⇒ (x + 149)(x – 151)



Question 15.

Let us factorise the following algebraic expressions:

(x2 – y2) (a2 – b2) + 4abxy


Answer:

Given, (x2 – y2)(a2 – b2) + 4abxy


⇒ ((x – y)(x + y)(a – b)(a + b)) + 4abxy


⇒ (ax)2 – (bx)2 – (ay)2 + (by)2 + 4abxy


⇒ (ax)2 + (by)2 + 4abxy – (bx)2 – (ay)2


⇒ (ax)2 + (by)2 + 2abxy + 2abxy – (bx)2 – (ay)2


[from Identity I, (x + y)2 = x2 + 2xy + y2 and from identity II, (x – y)2 = x2 – 2xy + y2]


⇒ (ax + by)2 – (ay – bx)2


⇒ ((ax + by) + (ay – bx))((ax + by) – (ay – bx))




Let Us Work Out 8.3
Question 1.

Let us factorize the following algebraic expressions:

t9 – 512


Answer:

We know, a3 – b3 = (a – b)(a2 + ab + b2)


Given,


t9 – 512


= (t3)3 – (8)3


= (t3 – 8) (t6 + 8t3 + 64)


= [(t)3 – (2)3] (t6 + 8t3 + 64)


= (t – 2) (t2 + 2t + 4) (t6 + 8t3 + 64)



Question 2.

Let us factorize the following algebraic expressions:

729p6 – q6


Answer:

We know, a2 – b2 = (a + b) (a – b)


Given,


729p6 – q6


= (27p3)2 – (q3)2


= (27p3 + q3) (27p3 – q3)


Now, a3 – b3 = (a – b)(a2 + ab + b2) and a3 + b3 = (a + b)(a2 – ab + b2)


= [(3p)3 + (q)3] [(3p)3 – (q)3]


= (3p + q)(3p2 – 3pq + q2) (3p – q)(3p2 + 3pq + q2)



Question 3.

Let us factorize the following algebraic expressions:

8(p – 3)3 + 343


Answer:

We know, a3 – b3 = (a – b)(a2 + ab + b2)


Given,


8(p – 3)3 + 343


= [8(p – 3)]3 + (7)3


= [8(p – 3) + 7] [{8(p – 3)}2 – 8(p – 3).7 + 72]


= (8p – 24 + 7) [64(p – 3)2 – 56(p – 3) + 49]


= (8p – 17) [64(p2 – 6p + 9) – 56p + 168 + 49]


= (8p – 17) [64p2 – 384p + 576 – 56p + 168 + 49]


= (8p – 17) [64p2 – 440p + 793]



Question 4.

Let us factorize the following algebraic expressions:



Answer:

We know, a3 + b3 = (a + b)(a2 – ab + b2)


Given,






Question 5.

Let us factorize the following algebraic expressions:

(2a3 – b3)3 – b9


Answer:

Given,


(2a3 – b3)3 – b9


We know, (x – y)3 = x3 – y3 – 3xy(x – y)


Therefore,


= (2a3)3 – (b3)3 – 3(2a3)(b3)(2a3 – b3) – b9


= 8a9 – b9 – 6a3b3(2a3 – b3) – b9


= 8a9 – 12a6b3 + 6a3b6 – 2b9


= 2a9 – 2b9 + 6a9 – 12a6b3 + 6a3b6


= 2(a9 – b9) + 6a3(a6 – 2a3b3 + b6)


Now, we know: x3 – y3 = (x – y)(x2 + xy + y2) and


(x – y)2 = x2 – 2xy + y2


= 2(a3 – b3)(a6 + a3b3 + b3) + 6a3(a3 – b3)2


= 2(a – b)(a2 + ab + b2)(a6 + a3b3 + b6) + 6a3(a – b)2(a2 + ab + b2)2


= 2(a – b)(a2 + ab + b2)(a6 + a3b3 + b6 + 3a3(a – b)(a2 + ab + b2))


= 2(a – b)(a2 + ab + b2)( a6 + a3b3 + b6 + 3a3(a3 + a2b + ab2 – a2b – ab2 – b3)


= 2(a – b)(a2 + ab + b2)( a6 + a3b3 + b6 + 3a3(a3 – b3) )


= 2(a – b)(a2 + ab + b2)( a6 + a3b3 + b6 + 3a6 – 3a3b3 )


= 2(a – b)(a2 + ab + b2)(4a6 – 2a3b3 + b6)



Question 6.

Let us factorize the following algebraic expressions:

AR3 – Ar3 + AR2h – Ar2h


Answer:

Given,


AR3 – Ar3 + AR2h – Ar2h


= A(R3 – r3 + R2h – r2h)


= A[(R3 – r3) + h(R2 – r2)]


We know, a3 – b3 = (a – b)(a2 + ab + b2) and a2 – b2 = (a + b)(a – b)


= A [(R – r) ( R2 + Rr + r2) + h (R + r)(R – r)]


= A(R – r) [R2 + Rr + r2 + h (R + r)]


= A(R – r) [R(R + r) + r2 + h(R + r)]


= A(R – r) (R + r) (R + r2 + h)



Question 7.

Let us factorize the following algebraic expressions:

a3 + 3a2b + 3ab2 + b3 – 8


Answer:

Given,


a3 + 3a2b + 3ab2 + b3 – 8


= (a + b)3 – (2)3


We know, a3 – b3 = (a – b)(a2 + ab + b2)


= (a + b – 2) [(a + b)2 + 2(a + b) + 22]


We know, (a + b)2 = a2 + 2ab + b2


= (a + b – 2) [a2 + 2ab + b2 + 2a + 2b + 4]



Question 8.

Let us factorize the following algebraic expressions:

32x4 – 500x


Answer:

32x4 – 500x


= 4x (8x3 – 125)


= 4x [(2x)3 – (5)3]


We know, a3 – b3 = (a – b)(a2 + ab + b2)


= 4x (2x – 5) [(2x)2 + 2x.5 + (5)2]


= 4x (2x – 5) (4x2 + 10x + 25)



Question 9.

Let us factorize the following algebraic expressions:

8a3 – b3 – 4ax + 2bx


Answer:

Given,


8a3 – b3 – 4ax + 2bx


= (2a)3 – (b)3 – 2x(2a – b)


We know, a3 – b3 = (a – b)(a2 + ab + b2)


= (2a – b) ((2a)2 + 2a.b + (b)2) – 2x(2a – b)


= (2a – b) (4a2 + 2ab + b2) – 2x(2a – b)


= (2a – b) (4a2 + 2ab + b2 – 2x)



Question 10.

Let us factorize the following algebraic expressions:

x3 – 6x2 + 12x – 35


Answer:

Given,


x3 – 6x2 + 12x – 35


= x3 – 35 – 6x2 + 12x


= x3 – 35 – 6x(x – 2)




Let Us Work Out 8.4
Question 1.

Let us factorise the following algebraic expressions:

x3 + y3 – 12xy + 64


Answer:

x3 + y3 – 12xy + 64


⇒ x3 + y3 + 43 – 12xy


⇒ x3 + y3 + 43 - 3×4xy …Equation (i)


We use the identity


a3 + b3 + c3 - 3abc = (a + b + c) (a2 + b2 + c2 - ab - bc - ca)


Using the above identity in Equation (i) we get


x3 + y3 + 43 - 3×4xy = (x + y + 4) (x2 + y2 + 16 - xy - 4y - 4x)


(x + y + 4) (x2 + y2 + 16 - xy - 4y - 4x)



Question 2.

Let us factorise the following algebraic expressions:

8x3 – y3 + 1 + 6xy


Answer:

8x3 – y3 + 1 + 6xy


⇒ (2x) 3 + (- y) 3 + 13 + 6xy


⇒ (2x) 3 + (- y) 3 + 13 - 3×(2x)×(- y)×1 …Equation(i)


We use the identity


a3 + b3 + c3 - 3abc = (a + b + c) (a2 + b2 + c2 - ab - bc - ca)


Using the above identity in Equation (i) we get


⇒ (2x) 3 + (- y) 3 + 13 - 3×(2x)×(- y) = (2x – y + 1) ((2x)2 + y2 + 12 + 2xy + y - 2x)


⇒ (2x) 3 + (- y) 3 + 13 - 3×(2x)×(- y) = (2x – y + 1) (4x2 + y2 + 1 + 2xy + y - 2x)


(2x – y + 1) (4x2 + y2 + 1 + 2xy + y - 2x)



Question 3.

Let us factorise the following algebraic expressions:

8a3 – 27b3 – 1 – 18ab


Answer:

8a3 – 27b3 – 1 – 18ab


⇒ (2a) 3 + (- 3b) 3 + (- 1)3 - 18ab


⇒ (2a) 3 + (- 3b) 3 + (- 1)3 - 3×(2a)×(- 3b)×(- 1) …Equation(i)


We use the identity


a3 + b3 + c3 - 3abc = (a + b + c) (a2 + b2 + c2 - ab - bc - ca)


Using the above identity in Equation (i) we get


⇒ (2a) 3 + (- 3b) 3 + (- 1)3 - 3×(2a)×(- 3b)×(- 1) = (2a - 3b - + 1) (4a2 + 9b2 + 1 + 6ab - 3b - 2a)


(2a - 3b - 1) (4a2 + 9b2 + 1 - 6ab + 3b + 2a)



Question 4.

Let us factorise the following algebraic expressions:

1 + 8x3 + 18xy – 27y3


Answer:

1 + 8x3 + 18xy – 27y3


⇒ 13 + (2x) 3 + (- 3y) 3 + 18xy


⇒ 13 + (2x) 3 + (- 3y) 3 + 3×(1)×(2x) (- 3y) …Equation(i)


We use the identity


a3 + b3 + c3 - 3abc = (a + b + c) (a2 + b2 + c2 - ab - bc - ca)


Using the above identity in Equation (i) we get


⇒ 13 + (2x) 3 + (- 3y) 3 + 3×(1)×(2x) (- 3y) = (1 + 2x - 3y) (1 + 4x2 + 9y2 - 2x + 6xy + 3y)


(1 + 2x - 3y) (1 + 4x2 + 9y2 - 2x + 6xy + 3y)



Question 5.

Let us factorise the following algebraic expressions:

(3a – 2b)3 + (2b – 5c)3 + (5c – 3a)3


Answer:

Let us consider (3a - 2b) = x, (2b - 5c) = y, (5c - 3a) = z


So x + y + z = 3a – 2b + 2b – 5c + 5c – 3a = 0


(3a – 2b) 3 + (2b – 5c) 3 + (5c – 3a) 3 = x3 + y3 + z3


Since x + y + z = 0, hence


x3 + y3 + z3 = 3xyz


⇒ (3a – 2b) 3 + (2b – 5c) 3 + (5c – 3a) 3 = 3×(3a – 2b)×(2b – 5c)× (5c – 3a)


3(3a – 2b) (2b – 5c) (5c – 3a)



Question 6.

Let us factorise the following algebraic expressions:

(2x – y)3 – (x + y)3 + (2y – x)3


Answer:

Let us consider (2x – y) = a, (2y – x) = b


a + b = (2x – y) + (2y – x) = x + y


So we can say that


(2x – y) 3 – (x + y) 3 + (2y – x) 3 = a3 + b3 – (a + b) 3 …Equation (i)


Using the identity of (a + b) 3 we get


(a + b) 3 = a3 + b3 + 3ab(a + b)


⇒ - 3ab(a + b) = a3 + b3 - (a + b)3


Using the above identity in Equation (i)


(2x – y) 3 – (x + y) 3 + (2y – x) 3 = - 3ab(a + b)


⇒ (2x – y) 3 – (x + y) 3 + (2y – x) 3 = - 3×(2x – y)× (2y – x)× (x + y)


⇒ (2x – y) 3 – (x + y) 3 + (2y – x) 3 = 3×(2x – y)× (x - 2y)× (x + y)


3(2x – y) (x - 2y) (x + y)



Question 7.

Let us factorise the following algebraic expressions:

a6 + 32a3 – 64


Answer:

a6 + 32a3 – 64


⇒ a6 + 8a3 – 64 + 24a3


⇒ (a2)3 + (2a) 3 + (- 4)3 - 3×(a2)×(2a)×(- 4) …Equation (i)


We use the identity


a3 + b3 + c3 - 3abc = (a + b + c) (a2 + b2 + c2 - ab - bc - ca)


Using the above identity in Equation (i) we get


⇒ (a2)3 + (2a) 3 + (- 4)3 - 3×(a2)×(2a)×(- 4) = (a2 + 2a - 4) (a4 + 4a2 + 16 - 2a3 + 8a + 4a2)


⇒ (a2 + 2a - 4) (a4 + 8a2 + 16 - 2a3 + 8a)


(a2 + 2a - 4) (a4 - 2a3 + 8a2 + 8a + 16)



Question 8.

Let us factorise the following algebraic expressions:

a6 – 18a3 + 125


Answer:

a6 – 18a3 + 125


⇒ a6 + 27a3 + 125 - 45a3


⇒ (a2)3 + (3a) 3 + 53 - 45a3


⇒ (a2)3 + (3a) 3 + 53 – 3× a2× 3a× 5 …Equation (i)


We use the identity


a3 + b3 + c3 - 3abc = (a + b + c) (a2 + b2 + c2 - ab - bc - ca)


Using the above identity in Equation (i) we get


⇒ (a2)3 + (3a) 3 + 53 – 3× a2× 3a× 5 = (a2 + 3a + 5) (a4 + 9a2 + 25 - 3a3 - 15a - 5a2)


⇒ (a2)3 + (3a) 3 + 53 – 3× a2× 3a× 5 = (a2 + 3a + 5) (a4 + 4a2 + 25 - 3a3 - 15a)


(a2 + 3a + 5) (a4 - 3a3 + 4a2 - 15a + 25)



Question 9.

Let us factorise the following algebraic expressions:

p3 (q – r)3 + q3 (r – p)3 + r3 (p – q)3


Answer:

p3 (q – r) 3 + q3 (r – p) 3 + r3 (p – q) 3


⇒ (pq – pr) 3 + (qr – pq) 3 + (pr – qr) 3


Let us consider pq – pr = a, qr – pq = b, pr – qr = c


a + b + c = pq – pr + qr – pq + pr – qr = 0


Since a + b + c = 0, hence


a3 + b3 + c3 = 3abc


⇒ p3 (q – r) 3 + q3 (r – p) 3 + r3 (p – q) 3 = 3pqr(q - r) (r – p) (p – q)


3pqr (q - r) (r – p) (p – q)



Question 10.

Let us factorise the following algebraic expressions:



Answer:



…Equation (i)


We use the identity


a3 + b3 + c3 - 3abc = (a + b + c) (a2 + b2 + c2 - ab - bc - ca)


Using the above identity in Equation (i) we get







Let Us Work Out 8.5
Question 1.

Let us factorise the following algebraic expressions:

(a + b)2 – 5a – 5b – 6


Answer:

The given expression can be rewritten as:


(a + b)2 – 5(a + b) + 6


Assume (a + b) = p,


⇒ p2 – 5p+ 6


⇒ p2 – 2p - 3p + 6


⇒ p(p - 2) -3(p - 2)


⇒ (p – 3)(p – 2)


On substituting the value of p, we get,


(a +b - 3)(a + b – 2)



Question 2.

Let us factorise the following algebraic expressions:

(x + 1) (x + 2) (3x – 1) (3x – 4) + 12


Answer:

The given expression can be rewritten as:


(x + 1)(3x – 1)(x + 2)(3x – 4) + 12


⇒ (3x2 – x + 3x - 1)(3x2 - 4x + 6x - 8) + 12


⇒ (3x2 + 2x - 1)(3x2 + 2x - 8) + 12


Let 3x2 + 2x = p


⇒ (p - 1)( p – 8) + 12


⇒ p2 – 9x + 8 + 12


⇒ p2 – 9p + 20


⇒ p2 – 5p – 4p + 20


⇒(p – 5)(p – 4)


On substituting the value of p, we get,


⇒ (3x2 + 2x – 5)(3x2 + 2x – 4)


⇒(3x2 + 5x – 3x – 5)(3x2 + 2x – 4)


⇒ (x(3x + 5) – 1(3x + 5))(3x2 + 2x – 4)


⇒ (x – 1)(3x + 5) (3x2 + 2x – 4)



Question 3.

Let us factorise the following algebraic expressions:

x(x2 – 1) (x + 2) – 8


Answer:

As we know that,


a2 – b2 = (a – b)(a + b)


The given expression can be rewritten as:


x(x + 1)(x – 1)(x + 2) – 8


⇒ (x2 + x)(x2 + x – 2) – 8


Let x2 + x = p


⇒ p(p – 2) – 8


⇒ p2 – 2p – 8


⇒ p2 – 4p + 2p – 8


⇒ p(p – 4) + 2(p – 4)


⇒ (p + 2)(p – 4)


On substituting the value of p, we get,


⇒ (x2 + x + 2)(x2 + x – 4)



Question 4.

Let us factorise the following algebraic expressions:

7(a2 + b2)2 – 15(a4 – b4) + 8 (a2 – b2)2


Answer:

As we know that,


x2 – y2 = (x – y)(x + y)


The given expression can be rewritten as:


7(a2 + b2)2 – 15(a2 + b2)(a2 – b2) + 8 (a2 – b2)2


Let (a2 + b2) = p and (a2 – b2) = q


⇒ 7p2 – 15pq + 8q2


⇒ 7p2 – 7pq – 8pq + 8q2


⇒ 7p(p – q) – 8q(p – q)


⇒ (7p – 8q)(p – q)


On substituting the value of p and q, we get,


⇒ (7(a2 + b2) - 8(a2 – b2))( a2 + b2 – a2 + b2)


⇒ (7a2 + 7b2 – 8a2 + 8b2) (2b2)


⇒ 2b2(15b2 – a2)



Question 5.

Let us factorise the following algebraic expressions:

(x2 – 1)2 + 8x(x2 + 1) + 19x2


Answer:

As we know that,


a2 – b2 = (a – b)(a + b)


The given expression can be rewritten as:


(x + 1)2(x – 1)2 + 8x(x2 + 1) + 19x2


Using (a – b)2 = a2 + b2 – 2ab, and


(a + b)2 = a2 + b2 + 2ab


⇒ (x2 + 1 + 2x)(x2 + 1 – 2x) + 8x(x2 + 1) + 19x2


Let x2 + 1 =p


⇒ (p + 2x)(p – 2x) + 8xp + 19x2


⇒ p2 – 4x2 + 8xp + 19x2


⇒ p2 + 8xp + 15x2


⇒ p2 + 3xp + 5xp + 15x2


⇒ p(p+3x) + 5x(p + 3x)


⇒ (p + 5x)(p + 3x)


On substituting the value of p, we get,


⇒ (x2 + 5x + 1)(x2 + 3x + 1)



Question 6.

Let us factorise the following algebraic expressions:

(a – 1)x2 – x – (a – 2)


Answer:

The given expression can be rewritten as:


(a – 1)x2 – x((a - 1) – (a - 2)) – (a – 2)


⇒ (a – 1)x(x – 1) + (a – 2)(x – 1)


⇒ (x – 1)(ax – x + a - 2)



Question 7.

Let us factorise the following algebraic expressions:

(a – 1)x2 + a2xy + (a + 1)y2


Answer:

Let p = (a – 1) and q = (a+1)


As we know that, a2 – b2 = (a – b)(a + b)


⇒ pq = a2 + 1


The given expression can be rewritten as:


px2 + pqxy + pq + qy2


⇒ px(x + qy) + y(x + qy)


⇒ (px + y)(x + qy)


On substituting the value of p and q, we get,


(ax – x + y)(x + ay + y)



Question 8.

Let us factorise the following algebraic expressions:

x2 – qx – p2 + 5pq – 6q2


Answer:

The given expression can be rewritten as:


X2 – qx – (p2 – 5pq + 6q2)


⇒ x(x – q) – (p2 – 2pq - 3pq + 6q2)


⇒ x2 – qx – (p – 2q)(p – 3q)


⇒ x2 + (p – 2q)x – (p – 3q)x – (p – 2q)(p – 3q)


⇒ (x – p + 2q)(x + p – 3q)



Question 9.

Let us factorise the following algebraic expressions:



Answer:

The given expression can be rewritten as:




As we know that, x2 + y2 - 2xy = (x – y)2



Let


⇒ 2p2 – p – 3


⇒ 2p2 – 3p + 2p – 3


⇒ p(2p – 3) + (2p – 3)


⇒ (p + 1) (2p – 3)


On substituting the value of p, we get,




Question 10.

Let us factorise the following algebraic expressions:

(x2 – x) y2 + y – (x2 + x)


Answer:

The given expression can be rewritten as:


x(x – 1)y2 + x2y – (x2 – 1)y – x(x+1)


⇒ x2 – xy2 + x2y – x2y + y – x2 – x


As we know that a2 – b2 = (a + b)(a – b)


⇒ xy[(x – 1)y + x] – (x + 1)[(x – 1)y + x]


⇒ (xy – x - 1)(xy + x – y)



Question 11.

If a2 – b2 = 11 × 9 and a & b are positive integers (a>b) then
A. a = 11, b =9

B. a = 33, b = 3

C. a = 10, b =1

D. a = 100, b =1


Answer:

Using the identity x2 – y2 = (x + y)(x – y)


⇒ (a + b)(a – b) = 11 × 9


Since 11 is a prime number therefore the factors can be equated,


⇒ a + b = 11


And a – b = 9


On solving above two equations we get,


a = 10 and b = 1


Question 12.

If then the value of a3 + b3 is
A. 1

B. a

C. b

D. 0


Answer:

The given equation on LCM reduces to:


a2 + b2 = ab


Also we know that,


a3 + b3 = (a + b)(a2 + b2 – ab)


On substituting the value of a2 + b2 in above equation, we get,


⇒ (a + b) (ab – ab)


⇒ 0


Question 13.

The value of 253 – 753 + 503 + 3 × 25 × 75 × 50 is
A. 150

B. 0

C. 25

D. 50


Answer:

The given expression can be rewritten as:


253 – 753 – (25 – 75)3 - 3(25)(75)(25 – 75)


As we know that,


(a – b)3 = a3 – b3 - 3ab(a - b)


⇒ a3 – b3 - (a – b)3 - 3ab(a - b) = 0


Since we can easily compare the above equation with the given expression by putting a = 25 and b = 75, the value of given expression is 0.


Question 14.

If a + b + c = 0, then the value of is
A. 0

B. 1

C. -1

D. 3


Answer:


We know the identity that, if a + b+ c = 0, then,


a3 + b3 + c3 = 3abc



Question 15.

If x2 – px + 12 = (x – 3) (x – a) is an identity, then the values of a and p are respectively.
A. a = 4, p = 7

B. a = 7, p = 4

C. a = 4, p = -7

D. a = -4, p = 7


Answer:

(x – 3) (x – a) = x2 – (3 + a)x + 3a


⇒ 3a = 12


⇒ a = 4


Also, p = 3 + a


⇒ p = 3 + 4 = 7


Question 16.

Short answer type questions:

Let us write the simplest value of


Answer:

Let p = (b2 – c2), q = (c2 – a2) and r = (a2 – b2)


Since p + q+ r = 0 and we have an identity that if,


x + y + z = 0, the x3 + y3 + z 3 = 3xyz


⇒ p3 + q3 + r3 = 3pqr


Similarly in denominator of given fraction, we see that sum of all the individual terms is equal to 0, so same identity can be applied in denominator as well.


The fractions reduces to:



Also as, x2 – y2 = (x - y)(x + y)




Question 17.

Short answer type questions:

Let us write the relation of a, b and c if a3 + b3 + c3 – 3abc = 0 and a + b + c ≠ 0.


Answer:

As we know from the identity that,


a3 + b3 + c3 – 3abc = (a + b + c)(a2 + b2 + c2 – ab – bc – ca)


Since a + b + c ≠ 0, that means,


a2 + b2 + c2 – ab – bc – ca = 0


Multiplying both sides by 2, we get,


a2 + b2 – 2ab + b2 + c2 – 2bc + a2 + c2 – 2ac = 0


⇒ (a – b)2 + (b – c)2 + (a – c)2 = 0


Now this is only possible if:


a – b = 0, b – c = 0 and a – c =0


⇒ a = b = c



Question 18.

Short answer type questions:

If a2 – b2 = 224 and a and b are negative integers (a < b), then let us write the values of a and b.


Answer:

224 can be written as 16 × 14


224 = (-16) × (-14)


Also using the identity, x2 – y2 = (x + y)(x – y)


⇒ a2 – b2 = (a + b)(a - b) = (-16) × (-14)


On equate the factors, as


a + b = -16 and (a – b) = -14 and solving these two equations, we get,


a = -15 and b = -1



Question 19.

Short answer type questions:

Let us write the value of (x – a)3 + (x – b)3 + (x – c)3 – 3(x – a) (x – b) (x – c) if 3x = a + b + c.


Answer:

As we know the identity that,


If l + m+ n = 0, then l3 + m3 + n3 – 3mnl = 0


Let p = (x - a), q = (x - b), r = (x – c)


p + q + r = (x – a + x – b + x – c) = 3x – a – b – c


also It is given 3x = a + b + c,


⇒ p + q + r = 0


⇒ p3 + q3 + r3 – 3pqr =0


On substituting the values of p, q and r in above equation, we get,


(x – a)3 + (x – b)3 + (x – c)3 – 3(x – a) (x – b) (x – c) = 0


∴ the required value is 0.



Question 20.

Short answer type questions:

Let us write the values of a and p if 2x2 + px + 6 = (2x– a) (x – 2) is an identity.


Answer:

(2x– a) (x – 2) = 2x2 – 4x – ax + 2a


⇒ 2x2 –x(a + 4) + 2a


Since the above expression is identical to 2x2 + px + 6, therefore their coefficients can be equated.


⇒ 2a = 6


⇒ a =3


Also, p = -(a + 4) = -7


∴ The values of a and p are 3 and -7 respectively.