: Given a function `f` and a specific `x`-value `x = c`, compute the slope of the
line tangent to `f` at `x = c`.
We denote this slope by `f'(c)`, and we say `f'(c)` is the derivative of `f` at `x = c`.
[`f'(c) = lim_(h->0) (f(c + h) - f(c))/h`]
With the applet above, we'll explore where this formula comes from. Note that
in the applet you can move two points on the
`x`-axis -- one with `x`-value `c`, and one with `x`-value `c + h`.