Differentiate given problems w.r.t.x.
cos(a cos x+b sin x),for some constants a and b.
Let y=cos(a cos x+b sin x)
Differentiating w.r.t x, we get
ddx(acosx+bsinx)=−sin(acosx+bsinx)ddx(acosx+bsinx)
=-sin(a cos x + b sin x){-a sin x + b cos x}
=(a sin x-b cos x) sin (a cos x + b sin x)