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

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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>

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