Решение во вложении:
-5у= -х - 15
-9/4x = - 18/5 , x≠0
( a/b=c/d ⇔ ad=bc)
-45=-72x
72x=45 /:9
8x=5 /:8
x=5/8
x=0,625
f(x)=cos²(x/4)-sin²(x/4)=cos(2*(x/4))=cos(x/2)
f'(x)=-sin(x/2)*(1/2)=-sin(x/2)/2=0
-sin(x/2)/2=0 |×(-2)
sin(x/2)=0
x/2=πn |×2
x=2πn.