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$f\left(x\right)=2\sin x-x$

a)

$f'\left(x\right)=2\cos x-1$

$f'\left(0\right)=2-1=1$

b)

$f'\left(x\right)=0$
$2\cos x-1=0$
$\cos x=\frac{1}{2}$
$x=\frac{\pi}{3}+n\cdot2\pi$

tai

$x=-\frac{\pi}{3}+n\cdot2\pi$

c)

$f'\left(3\pi\right)=2\cos3\pi-1=-3$