Limit definition of Derivative of a function at x=a : :
lim (f(a+h)-f(a))/h =
h->0
From limit definition i need just h in the denominator, so i can substitute, h = x-pi/4 as
x -> pi/4, h ->0
also x = pi/4+h
Comparing this with limit definition f(a+h) and tan (pi/4+h), we can figure out , f(x) = tan x , and a = pi/4. I hope that this is the answer that you were looking for and it has helped you.
