Question

can someone explain how to do this? plz

Answer 1

Well, first you need to study what's under the square root.

(1-cos)*(1+cos) - which is 1 - cos^2.

It's a formula that's well known: (a+b)(a-b)=a^2-b^2 but if you don't know it then you might as well multiply and add each element there and see that you get to the same result.

However, 1-cos^2 = sin^2 because sin^2+cos^2=1 (fundamental theorem).

Ergo, you now have sqrt(sin^2) which is sin.