This time we get sub-intervals of length 5. We can see our answer to (c) from the picture.
Since a straight line has no concavity, the trapezoid rule gives the exact value of the integral. We can verify this by finding the areas of the triangles above and below the x-axis andcalculating the value of the integral exactly. The line crosses the x-axis when x = 6. The big triangle has base 16 and "height" f (-10) = -8, so its weighted area is -64.
The little triangle has base 4 and height f (10) = 2, so its weighted area is 4. Combining these, we get
which confirms that the trapezoid rule gave us the exact value of the integral.