Question

If x be the real number such that $x^3+4x=8$, then the value of the expression $x^7+64x^2$ is

• A. $124$
• B. $125$
• C. $128$
• D. $132$

Answer 1

The correct option is C $128$

$x$ is the real number such that $x^3+4x=8$ .... $\left(i\right)$

Now,

$x^7+64x^2$

$=x\times {\left(x^3\right)}^2+64x^2$

$=x\times {(8-4x)}^2+64x^2$ ..... Using $\left(i\right)$

$=64x+16x^3$

$=16(4x+x^3)=16\times 8=128$