Which trigonometric identity was introduced in this unit?
a) sin^2 x= 1 + cos^2 x
b) (sin x + cos x)^2 = 1
c) tan x = sin x/cos x
d) tan x= cos x/sin x

One way to determine if the function is a trigonometric identity or not is to use a calculator on the choices. Choice A does not fit. Choice B should be (sin x)^2 + (cos x)^2 to become equal to 1. Choice D> should be  cot x, the inverse of tan x to become equal to cos x/ sin x. Hence, answer is C>