Calculus consists of those problems in mathematics that can be solved with the following basic approach:
Approximate and take a limit!
The simplest, standard, geometric example of this process is the approximation of the slope of a tangent line with a secant line. Here's an illustration of this basic idea.
Choose $$f$$: $$f(x)=x^2$$ $$f(x)=\frac{1}{3}e^x$$ $$f(x)=\sin(2x^2)+x/4$$
 Show secant line
 $$h$$ = secant slope = tangent slope =