$f(x)=\cos(ax+b)$ find $f'(x)$ , $f''(x)$, $f'''(x)$, $f''''(x)$ and give a general form for $f^{(n)}( x)$ where there s cosinus

Question

I found \begin{align*} f'(x)&=-a\cos(\pi/2-ax-b) \\ f''(x)&= a^2 \cos(ax+b) \\ f'''(x)&=-a^3 \cos(\pi/2-ax-b) \\ f''''(x) &= a^4 \cos(ax+b) \\ f^{(n)} (x)&= (-1)^n a^n \cos(\dotsb +((-1)^n))(ax+b) \end{align*} I son t know what about the $\pi/2$ please help thanks