$f(x) = \frac{32}{1029}x^3 - \frac{16}{35}x^2 + 2x$, $D = [1,9]$
Area $= \int_1^9 f(x)\,dx = \frac{34416}{1715} \approx 20.0676$
Riemann approximation
≈
19.9681
Number of slices: