A few trig problems- help please (double angle and simplifying trig expressions)

Hey, I have been trying to figure out how to do these problems for quite some time, but I'm stuck. Help is very much appreciated.

Verify:

2- ((sin^2(x)/tan^2(x))= sin^2x-1

I really cannot figure this out.

Also, how do you solve this problem for theta? (this is one where I'm supposed to find the reference angle):

2sin2u+sin=0

and last thing, how do you find the cot(M+N)? Is it the same as the sum formula for cos but under 1?

Thank you so much!