Hello, can someone please explain how to do this?   Compute the Riemann sum for f(x) = 21 – x^2 on [1,4] using the partition P = {1,2, 2.5, 3, 4} and - the left endpoint of each subinterval - the midpoint of each subinterval - lastly, calculate the Riemann sum using a partition with six equal-width subintervals and the right endpoint of each subinterval

Hello, can someone please show me how to illustrate the midpoint riemann sum with a graph?   Compute the Riemann sum for f(x) = 21 – x^2 on [1,4] using the partition P = {1,2, 2.5, 3, 4} and - the left endpoint of each subinterval - the midpoint of each subinterval - lastly, calculate the Riemann sum using a partition with six equal-width subintervals and the right endpoint of each subinterval

How do we calculate the lower Riemann sum L(f,P) and the upper Riemann sum U(f,P) given the partition P = {0, 1, 2, 3} of the interval [0, 3].