Найти производную функции $f(x) = (1-5x)^{20}$.

$u=1-5x$, $u'=-5$. $$ f'(x) = 20u^{19}\cdot u' = 20(1-5x)^{19}\cdot(-5) = -100(1-5x)^{19}. $$

$$ \boxed{f'(x) = -100(1-5x)^{19}} $$