Limit definition of the Derivative of a function:\(\lim_{h \to 0} \frac{f(x+h)-f(x)}{h}\) At x = a:\(\lim_{h \to0} \frac{f(a+h)-f(a)}{h}\)
If: h = x -π/4, we will have:\(\lim_{h \to \pi /4} \frac{tan ( \pi /4 +h)-1}{h}\) When we compare it with a limit definition:
f( x )= tan x
f( a ) = 1
tan ( a ) = 1, a = π/4
