That's the question I can't do. It would be so much easier if it was a cos squared!
I think I have to use a substitution but i'm not quite sure what it should be.
$\displaystyle \sqrt{1-\cos{\theta}} $
$\displaystyle = \displaystyle\sqrt{\underbrace{1-\cos^2{\frac{\theta}{2}}}_{\sin^2{\frac{\theta}{2} }}+\sin^2{\frac{\theta}{2}}}$
$\displaystyle = \sqrt{2\sin^2{\frac{\theta}{2}}}$
$\displaystyle = \sqrt{2}\sin{\frac{\theta}{2}}$