SSC CGL 17 Aug 2017 Shift 2 Paper

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Question : 62 of 100

Marks: +1, -0

If

a^{3}+b^{3}=19

and

a b=-6,

then what is the value of

a+b ?

5
7
1
-5

Solution:

a^{3}+b^{3}=19

and

a b=-6,

By formula

a^{3}+b^{3}=(a+b)^{3}-3 a b(a+b)

⇒

19=(a+b)^{3}+18(a+b)

Let

a+b=x

⇒ x^{3}+18 x-19=0

Putting x = 1 in the above equation,

1^{3}+18-19=0 ⇒ 0=0

∴

(a+b)=x=1

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