MATLAB: I’m getting wrong answer


I'm trying to calculate the volume under x^2+ 1 curve when the curve is rotated around the x axis.This integration is from 0 to 3 with x incrementing by 0.1 using disk method.I did is the following way: l = 0.1;
sum = 0
for index = 1:30;
x = (index-1)*0.1;
y = pi*((x^2+1)^2)*l;
sum = sum + y;
clc; I'm getting volume 203.418093.However, the calculator integration gives volume 218.6548.

Best Answer

  • In each of your 30 disks you are using the radius at its smaller end, so of course your answer will be smaller than the true volume. You will get a more accurate answer if you select the central radius for each disk. You will also get more accuracy if you cut the volume into more than 30 disks. Your "calculator" accuracy is, of course, achieved by using the methods of calculus, which would be the smart way in this problem.