If x^4 + 1/x^4 = 527, then what it the value of x^3 + 1/x^3 ?

Question

If  x^4 + \frac{1}{x^4} = 527 , then what it the value of  x^3 + \frac{1}{x^3}  ?

A. 70
B. 80
C. 90
D. 110
E. 120

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GMATinsight · Answer

x^4 + \frac{1}{x^4} = 527 
Adding 2 both sides
ie..  x^4 + \frac{1}{x^4} + 2 x^2*\frac{1}{x^2} = 527+2

i.e.   ^2 = 529 = 23^2

x^2+\frac{1}{x^2} = 23

Also,  x^2+\frac{1}{x^2}+2*x*\frac{1}{x} = 23+2

i.e.  x+ (\frac{1}{x}) = 5

x^3 + \frac{1}{x^3} = (x+\frac{1}{x})(x^2+(\frac{1}{x^2})-

x^3 + \frac{1}{x^3} = (5)(23-1) = 110

Answer: Option D

nisen20 · Answer

this part is ok.
 
here we got a problem.
 
so there should be two values

Regor60 · Answer

Yes, but -110 is not among the answer choices

Posted from my mobile device

braddouglas10 · Answer

Bunuel

Do you have a different approach?

kevincan · Answer

Replace is with could be or make it clear that x is positive , and it’s a very nice question !

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