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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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