Question
If $3 x-\frac{1}{3 x}=5$, find : $81 x^4+\frac{1}{81 x^4}$
✓
Answer
$81 x^4+\frac{1}{81 x^4}$
$=\left(9 x^2+\frac{1}{9 x^2}\right)^2-2$
$=(27)^2-2$
$=729-2$
$=727$
$=\left(9 x^2+\frac{1}{9 x^2}\right)^2-2$
$=(27)^2-2$
$=729-2$
$=727$
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