Question

f(x)=0,2cosx , find f'(x)

Answer 1

0,2 is a number there is a number or you did a typo? :)
Anyway, 0,2 is a constant, so you can take it away and derive cosx.
f' cosx => -senx

Stick in the constant back you get..
-0,2sinx

Answer 2

It's a number . :) thanks!