Find the derivative of $f(x)$ = $(x^2 + 1)(x^3 − 3x) $

The correct answer is $5x^4−6x^2−3$

As per Product rule,

$\dfrac{d}{dx} (f(x)g(x))$ = $f(x)g′(x) + f′(x)g(x)$

$∴ for f(x)$ = $(x^2 + 1)(x^3 − 3x) $

$f′(x)$ = $(x^2+1)(3x^2−3)$ + $(2x)(x^3−3x)$
= $3x^4−3x^2+3x^2−3$ + $2x^4−6x^2$
= $5x^4−6x^2−3$