Will someone help me solve this? I tried to solve many times and keep getting 0 as the answer. Wolfram Alpha says the answer should be (secy)^2.
Without using any trigonometric identities, find
lim x->0 [tan(x + y) - tan y]/x
Hint: Relate the given limit to the definition of the derivative of an appropriate function of y.
I used tan(x + y) = (tan x + tan y)/(1 - tan x tan y). I still got 0 as the answer.