The slope of the tangent line to the graph of f at the point (c, f(c)) is called
the slope of the graph of f at x=c.
It is equal to the derivative of f at the point (c, f(c)).
The process of finding the derivative of a function is called differentiation.
It is very close concept with limits.
The derivative of f at x is given by f'(x) = lim((f(x+Δx)-f(x))/Δx, Δx=0)
provided the limit exists (See Figure 06).
f is differentiable at x = c when f has same values between the derivatives
from the left and from the right and f is continuous at x = c.
