Question

Why is the derivative of sinx cosx

Answer 1

product rule for this one
\[\left(fg\right)'-f'g+g'f\] with
\[f(x)=\sin(x), f'(x)=\cos(x), g(x)=\cos(x), g'(x)=-\sin(x)\]

Answer 2

oops i meant\[(fg)'=f'g+g'f\]