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Algebra Level 3

If x 1 x = 2 x - \frac{1}{x} = 2 , then find the value of x 3 1 x 3 x^3 - \frac{1}{x^3} .


The answer is 14.

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2 solutions

Alex Delhumeau
May 19, 2015

If x 1 x = 2 x-\frac{1}{x}=2 , then ( x 1 x ) 3 = 8 (x-\frac{1}{x})^3=8 .

( x 1 x ) 3 = x 3 1 x 3 3 ( x 1 x ) = x 3 1 x 3 6 = 8 (x-\frac{1}{x})^3=x^3-\frac{1}{x^3}-3(x-\frac{1}{x})=x^3-\frac{1}{x^3}-6=8 .

\implies x 3 1 x 3 = 14 x^3-\frac{1}{x^3}=\boxed{\large{14}} .

Munem Shahriar
May 19, 2017

It was given, x - 1 x \frac{1}{x} = 2

x³ - 1 x ³ \frac{1}{x³}

= (x)² - ( 1 x \frac{1}{x}

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

= (2)³ + 3 × 2

= 8 + 6

= 14 (Answer)

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