Home | 18.022 | Chapter 11 | Section 11.4

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You want to integrate over the triangle bounded by the line x = y, x = 2 and the axis, y = 0.
This can be written as the integral first over y from 0 to x, and then from x from 0 to 2. If we want to integrate first over x, then we must integrate from y to 2 over x and from 0 to 2 over y.
Many beginners fall into the trap of using the same limits after reversing the order of integration. This is an error.
The region 0 < y < x, 0 < x < 2 is very different from the region 0 < x < y, 0 < y < 2. Obviously in the former y is always less than x and in the latter y is always bigger than x. Do not make this mistake.