Question

how do you do this? sin(x/2)=1-cos x

Answer 1

use the property-
$$\cos(2x)=1-2\sin^2x$$

Answer 2

you mean \[\large \sin^2 (\dfrac{x}{2})?\]

Answer 3

that is for $$\cos(x)$$

Answer 4

I have mentioned it for $$\cos(2x)$$