Showing point in f(x) such that f'(x)=0 exists

45 Views Asked by At

let $f(x)=x^4 + \sin x$

Show that there exists $x \in (-2,2)$ such that $f'(x) = 0$.

2

There are 2 best solutions below

0
On

Hint: $f(\frac{\pi}{2}), f(-\frac{\pi}{2})> 0$, $f(0)=0$.

1
On

Guide: Show that $$ f'(2)>0\quad f'(-2)<0 $$ and conclude using the intermediate value theorem.