Finding other orders of triple integrals.

integrationmultivariable-calculus

I find it insanely hard to imagine and change the order of integration for triple integrals. I have this problem for example. $\int_{0}^{2}\int_{0}^{y^3}\int_{0}^{y^2}f(x,y,z) dzdxdy$ So it is easy to change this to dzdydx because that only looking at the area at the bottom. However, changing it to anything else seems very hard, and requires sketching the volume. Could anyone tell me how to approach this problem, and problems like this in the future?

Thank you.

Best Answer

I'm going to try to expand on @SangchulLee's answer a little bit.

You think through the original problem by realizing that our 3-dimensional region projects down to the region in the $xy$-plane between the $y$-axis and $x=y^3$ [or, equivalently, $y=x^{1/3}$]. In $\Bbb R^3$, $x=y^3$ is a vertical cylinder over this plane curve. For each point in that plane region, a vertical line enters the region at $z=0$ and exits at $z=y^2$. So this surface is a parabolic cylinder, with lines parallel to the $x$-axis. Here's a sketch with the assistance of Mathematica:

3DRegion

Indeed, the region lies over the rectangle $R=[0,8]\times [0,4]$ in the $xz$-plane, and the two cylinders intersect along the crease $y=z^{1/2}=x^{1/3}$, which projects to the curve $C$ given by $x^2=z^3$ in the $xz$-plane. If we take a point in $R$, we want to know when a line parallel to the $y$-axis enters and exits our region. If we're below $C$ in $R$, then $z^3<x^2$ and the line enters at $y=x^{1/3}$ and exits at $y=2$. (As a check, if $y\ge x^{1/3}$, then $y\ge z^{1/2}$ as well in this range, so we're "inside" both cylinders.) If we're above $C$ in $R$, then $z^3>x^2$ and the line enters at $y=z^{1/2}$ and exits at $y=2$.

Thus, we end up with the following iterated integral in the order $dy\,dz\,dx$: $$\int_0^8\int_0^{x^{2/3}}\int_{x^{1/3}}^2 dy\,dz\,dx + \int_0^8\int_{x^{2/3}}^4\int_{z^{1/2}}^2 dy\,dz\,dx.$$

(By the way, you should be able to set up the limits in the orders $dx\,dz\,dy$ or $dx\,dy\,dz$ much more easily.)