(1 point) a) The rectangles in the graph below illustrate a| left endpoint . Riemann sum for f(x) = x2 [4, 8]. 11 on the interval The value of this Riemann sum is 6.5 , and this Riemann sum is an underestimate of * the area of the region enclosed by y = f(x), the x-axis, and the vertical lines x = 4 and x = 8. 7 6 5 3 2 1 X 2 3 5 6 7 8 Left endpoint Riemann sum for y = , on [4, 8]