Which equation is not an identity?

tan(–t) = –tan t

tan t(csc t + cot t) = sec t + 1

sin tcot t = cos t

cos2t − sin2t = –1

Question

Which equation is not an identity?

tan(–t) = –tan t

tan t(csc t + cot t) = sec t + 1

sin tcot t = cos t

cos2t − sin2t = –1

anonymous · Answer

@Shalante @oldrin.bataku

anonymous · Answer

i feel like it is d but in like 50% sure

anonymous · Answer

@UsukiDoll

usukidoll · Answer

thinking...

usukidoll · Answer

I've seen $$\cos^2t-\sin^2t $$ before. I think it was one of the identities for cos2t

anonymous · Answer

okay so d is right?

usukidoll · Answer

yeah... 

there's also another identity 
which is 
$$\large \cos^2t+\sin^2t=1 $$
but rearranging them a bit gives me 

$$\large \cos^2t=1-\sin^2t $$
$$\large \sin^2t=1-\cos^2t $$

anonymous · Answer

d was right lol this stuff is so confusing

usukidoll · Answer

|dw:1438846787051:dw|

usukidoll · Answer

one big huge trig list........and parts of it exist beyond trig.

anonymous · Answer

okay i posted another question do you think you could help me on that one as well?