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If x−1x=10x - \frac{1}{x} = 10x−x1​=10, then the value of x3−1x3x^3 - \frac{1}{x^3}x3−x31​ is:​
Question

If x1x=10x - \frac{1}{x} = 10, then the value of x31x3x^3 - \frac{1}{x^3} is:​

A.

986

B.

1030

C.

992

D.

1386

Correct option is B

Given:

x1x=10x-\dfrac{1}{x}=10​​

Find x31x3x^{3}-\dfrac{1}{x^{3}}​​​​

Solution:

(x1x)2=x2+1x22(x-\frac{1}{x})^{2}=x^{2}+\frac{1}{x^{2}}-2 ​​

x2+1x2=102+2=102x^{2}+\frac{1}{x^{2}}=10^{2}+2=102​​

x31x3=(x1x)(x2+1x2+1)=10(102+1)=10×103=1030x^{3}-\frac{1}{x^{3}}=(x-\frac{1}{x})\left(x^{2}+\frac{1}{x^{2}}+1\right)=10(102+1)=10\times 103=1030​​

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