для функции f(x)=5cos x вычислите f'(-3π/4)

f(x)=5cosx;f¹(x)=5·(-sinx)=-5sinx;f¹(-3π/4)=-5·sin(-3π/4)=-5·(-sin(3π/4)=5·sin(π-π/4)=5·sinπ/4=5·√2/2;