# FAQs For Lab 4

• Calculations
• Graph and Trendline

• Calculations
• My trapezoidal rule (number 1) and integral (number 4) gave different answers. What's wrong?

Your answers should be different, but not by a large amount. If they are very far off, try to determine which one must be incorrect. (You could for example easily come up with an upper limit or lower limit to the entire volume: just use one cylinder with a radius at the top or the bottom.)

Then check the following:
• Are you integrating the right function? (In both cases, you want pi times r squared. In the first case, you will need to convert the diameters in inches to radii in feet. In the second, you will use a formula for d2 and convert it to r2.)
• Did you use the right limits? For the trapezoidal rule, did you start and end at the right place on the table? For the integral, did you use the correct h values?
• If your integral seems wrong, does your formula for d2 look right? (Try a couple of values and make sure they are close to what is in the table.)
• If your trapezoidal rule is wrong, double check that you remembered everything needed for the trapezoidal rule (the delta x, the 1/2, the constants...)

• What's B? What's H?

B is the diameter at breast height and H is the total height of the tree. Both are discussed on the first page of the lab under "Introductory Material and Warm-up." The B and H for the particular tree discussed in Part I are given in the first paragraph above the table in Part I.

• How do I convert d2 to r2?

Since d/12 is the diameter in feet, and d/24 is half of this. So the radius in square inches, r2 is d2/242.

• Graph and Trendline
• How do I create a trendline?

Trendlines are created by selecting a chart and choosing "Add Trendline" from the menu. Instructions are provided in the Online Excel Help (see "Adding a Trendline").

Note: Be sure to choose a graph with markers only and no lines connecting them.