Question

cos^2xcos^2y+sin^2xsin^2y+sin^2xcos^2y+sin^2ycos^2x = 1

Answer 1

Where are you getting stuck?

Answer 2

I got up to 1-sin^2x - sin^2y + sin^2xsin^y +sin^2x-sin^2xsin^2y + sin^2y-sin^2ysin^x. I don't know if that's right or wrong.