So what if you didn't cover it? Be ahead of your class and derive the half-angle identities. And you can do it using your double angle identities. Attempt it for abit, and if you reach to it or hit a dead-end, you can go here:
http://math.ucsd.edu/~wgarner/math4c.../halfangle.htm
Using the double angle identity for tan to find tan(x/2) is just complicated and prone to error.