I need to demonstrate that this is true for my maths class, but I don't have ANY idea of how to do it. Help please?

It's been a looooong time since I did anything like this, but it is the same as any other math problem of this type. You need to simplify it so that you can see that the statement is true.The left side is a fraction over a fraction, so I would start there working with it algebraicly. You probably know some trigonometric substitutions so there's also that to try.

Try starting by multiplying boths sides by (sen alpha)^2 so that you have 1/1 on the right. Then I would tackle that monster on the left.

I noticed that also, but the beauty of it is you could have substituted foo or coconuts and it still generally works.