Question 136157
Given: cos^2xcos^2y+sin^2xsin^2y+sin^2xcos^2y+sin^2ycos^2x=1
We know that cos^2x + sin^2x = 1

cos^2xcos^2y+sin^2xsin^2y+sin^2xcos^2y+sin^2ycos^2x=1
cos^2xcos^2y+sin^2xcos^2y+sin^2xsin^2y+sin^2ycos^2x=1
(cos^2x + sin^2x)cos^2y+(sin^2x +cos^2x)sin^2y=1
(1)cos^2y+(1)sin^2y=1
cos^2y + sin^2y = 1
1 = 1   True!