I don't know identities but I've learned sin^2(@)+cos^2(@)=1. Try doing the tagent with sines and cosines.
tan(@) = 1/2 = sin(@)/cos(@)
Square both sides, and multiply acorss to get cos^2(@) = 4sin^2(@). Then add a sine to both sides to get cos^2(@)+sin^2(@)=5sin^2(@)=1. Then sin^2(@)=1/5. Do the square roots to get the answer.