Which of the following are antiderivatives of f(x)=sinxcosx?

Question

I f(x)=(sin^2* x)/2
II f(x)=(cos^2* x)/2
III f(x)=-cos(2x)/4

Answer 1

Why don't you in your head take the derivate of the answers?

d sin^2 x/2= 1/2 sinx cosx * 2

Answer 2

1 and 3