If \sqrtx +1/\sqrtx = \sqrt7 then x^3 + 1/x^3 is equal to:

SSC CHSL Model Paper 19

Question : 60 of 99

Marks: +1, -0

√x +1/√x = √7

then

x^3 + 1/x^3

is equal to:

Solution:

√x +1/√x = √7

⇒

(√x +1/√x )^2= (√7)^2

⇒

x+1/5=5

x^3 + 1/x^3=(x+1/x)^3

-3×x×1/x(x+1/x)

= (5)^3 - 3(5)

= 125-15= 110

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