Hi, i'm struggling on this question and would appreciate some hints as to what I do next.

Q: Given that x=tan(y/2), prove that dy/dx=2/1+x^2

so far i have got this;

dx/dy=[sec^2(y/2)]/2 dy/dx= 2/[cos^2(y/2)] = 2/1-sin^2(y/2)

i think i need to change sin^2(y/2) to tan^2(y/2) which will give me the x^2, unfortunately i can't think how. presumably the minus sign i have will become a plus as you can't square x and get a negative answer.