The rule says that if the first derivative(x) > 0 then it is increasing at that point and if the first derivative(x) < 0 then it is decreasing at that point.

I analyzed function f(x)=4*(sin(x))^2

f'=8*sin(x)*cos(x)

The function f(x) should increase at 150 degrees but the first derivative at 150 is < 0. Graph shows that this function increases at 150 degrees.

Why does not the first derivative show it ?