Hi

You don't need to take the derivative

y=cos(x) + sin(x)=sqrt(2) (sqrt(2)/2 . cos(x) + sqrt(2)/2 . sin(x)) = sqrt(2) (sin(pi/4) . cos(x) + cos(pi/4) . sin(x)) = sqrt(2) sin(x+pi/4)

Otherwise with the derivative

y'=cos(x) - sin(x)

y'=0 when x= pi/4 (mod 2pi) or x=5pi/4 (mod 2pi)

y(pi/4) = sqrt(2)

y(5pi/4) = -sqrt(2)

The problem is known. Admins are trying to fix it.