M1 - Force on pulley

I've seen this question come up a few times on past papers and on Arsey's model answers he uses the formula 2TCos((90 - alpha)/2) where alpha is the angle between the horizontal and the inclined plane.

The formula works but I don't really understand where its come from? Can someone please explain this for me? (I don't like using formulas that I don't understand). I have attached the 2015 question as an example. (Tension is 6.53N, acceleration is 3.27 ms-2).

Also, is there a way to work out the force from 'first principles' (for want of a better word) just in case they ask a question where the formula can't directly be used? ; ie work it out using a vector triangle, cosine rule etc?

Thanks in advance.

Watch this.

Thanks - got it now.