関連ワード:  sec^2x-tan^2x formula    sec 2x tan 2x formula    sec 2x tan 2x identity    sec 2x tan 2x 1 formula    sec 2x tan 2x trig identity    sec 2x tan 2x 1 identity    what is sec^2x-tan^2x    why does 1+tan^2x=sec^2x    sec^2x formula