Consider the graph below.

If there are n vertical strips then there are n+1 vertical lines(ordinates) bounding them.

The limits of the integral are between a and b, and each vertical line has length y₁ y₂ y₃... y_n+1

Therefore in terms of the all the vertical strips, the integral is given by:

approx. integral = (strip width) x (average of first and last y-values, plus the sum of all y values between the second and second-last value)

State the values of y for the graph y = x²  from 3 to 8.