let $f(x)=x^4 + \sin x$
Show that there exists $x \in (-2,2)$ such that $f'(x) = 0$.
Hint: $f(\frac{\pi}{2}), f(-\frac{\pi}{2})> 0$, $f(0)=0$.
Guide: Show that $$ f'(2)>0\quad f'(-2)<0 $$ and conclude using the intermediate value theorem.
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Hint: $f(\frac{\pi}{2}), f(-\frac{\pi}{2})> 0$, $f(0)=0$.