The textbook states something along the lines as prove the identity. 1 - ((sin^2x)/(1+cos x)) = cos x If you want you an work this out algebraically relatively easily to get cos x = cos x. But what if you put pi back into the original equation? You get 1 - undefined = cos x. So I graphed the two & the graphs looked the same, except the left hand side had a hole at odd multiples of pi. I showed the professor & asked him if it was still an identity. He looked in the solutions manual & its marked an identity & said he'd have to look into it further. Its been a couple of days & was just curious, it just doesn't seem to be a true a identity. What do other people think?