Question

Which equation is not an identity?

a. tan(–t) = –tan t
b. tan t(csc t + cot t) = sec t + 1
c. sin tcot t = cos t
d. cos2t − sin2t = –1

Answer 1

The best and most correct answer among the choices provided by the question is the fourth choice "cos2t − sin2t = –1"


I'd go with d. Let t = 0 and you'd get 1 - 0 = -1 which is false


I hope my answer has come to your help. God bless and have a nice day ahead!