SSC CGL Model Paper 2

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Question : 62 of 100

Marks: +1, -0

If a - b = 4 and ab = -3, then what is the value of

a^3 - b^3?

21
28
23
-20

Solution:

Given, (a - b) = 4, ab = -3

(a-b)^2=a^2+b^2-2ab

⇒

(4 )^2 = a^2 + b^2 - 2 (-3 )

⇒

1 6 = a^2 + b^2 + 6

∴

a^2 +b^2=10

⇒

a^3 - b^3 = (a - b) (a^2 + b^2 + ab)

⇒

a^3 - b^3 = 4 [10 + (-3 )]

⇒

a^3 - b ^3 = 4 (1 0 -3 )

∴

a^3 - b ^3 = 4 ×7 =28

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