Use a Riemann sum to approximate the area under the graph of f(x) on the given interval with the selected points as specified.
0 ≤ x ≤ 8, n = 4, Midpoint Rectangle Approximation Method
see your other post ...
http://www.mathhelpforum.com/math-he...emann-sum.html
If you are not given a formula for f(x) then either get the values off a graph, as skeeter suggested, if you have one, or, if you have no information about f at all, just write your formula in terms of f(1), f(3), f(5), and f(7), leaving those instead of specific values.