The slope of the secant line for the graph $y = f(x)$ between the
points
$(a, f(a))$ and
$(b, f(b))$,
is
$$slope~of~secant~line = \frac{f(b) - f(a)}{b - a} = \frac{\Delta y}{\Delta x}$$
images from
http://fym.asu.edu/~mat210_online/mat210_web/lessons/Ch1/1_5/skydiver_calc.jpg,
http://fym.asu.edu/~mat210_online/mat210_web/lessons/Ch1/1_5/secant1.gif,
http://www.blc.edu/fac/rbuelow/calc/secant.jpg,
http://mathdemos.gcsu.edu/mathdemos/escalator/avgratefig1.gif,
http://www.sagemath.org/calctut/pix-calctut/tangent03.png,
