I have differentiated t and got dt/dx = 1/2 sec^2(x/2) and then rearranged to get dx = 2dt/sec^2(x/2). So I know dx = 2dt/(1+t^2) but I'm not sure what to do with the sinx.
I don't know what's officially in any exam board, but if you are using that substitution you should work out how and remember how to express sin x, cos x and tan x in terms of t.
Thanks I appreciate it. Do you know where I could find the proofs for these expressions? I find it easier to remember them if I know how they are derived.