Which of the following are identities? check all that apply. a. cot2x + 1 = csc2x b. tan2x = 1 - sec2x c. sin2x = 1 + cos2x d. sin2x + cos2x = 1

I'll assume those are squares.  We know D is an identity:



Dividing through by



That's A.

Dividing the original through by



Not quite B, wrong sign on tangent.  

C has the wrong sign on cosine squared as well.

Identities: A & D