Polar Integration 361D79
1. **Problem Statement:**
Evaluate the triple integral $$\int_0^2 \int_0^{\sqrt{4-x^2}} \int_{-5+x^2+y^2}^{5-x^2-y^2} x \, dz \, dy \, dx$$ using polar coordinates, where the upper limit in the innermost integral is changed from $$3 - x^2 - y^2$$ to $$5 - x^2 - y^2$$ to simplify the problem.
2. **Understanding the integral:**
The integral is over the volume bounded by:
- $$x$$ from 0 to 2,
- $$y$$ from 0 to $$\sqrt{4 - x^2}$$ (which describes a quarter circle of radius 2 in the first quadrant),
- $$z$$ from $$-5 + x^2 + y^2$$ to $$5 - x^2 - y^2$$.
3. **Convert to polar coordinates:**
Recall the transformations:
$$x = r \cos \theta$$
$$y = r \sin \theta$$
$$dx \, dy = r \, dr \, d\theta$$
The region in the xy-plane is a quarter circle of radius 2 in the first quadrant, so:
$$0 \leq r \leq 2$$
$$0 \leq \theta \leq \frac{\pi}{2}$$
4. **Rewrite the limits and integrand:**
- The integrand is $$x = r \cos \theta$$.
- The limits for $$z$$ are from $$-5 + r^2$$ to $$5 - r^2$$.
5. **Set up the integral in polar coordinates:**
$$\int_{\theta=0}^{\pi/2} \int_{r=0}^2 \int_{z=-5+r^2}^{5-r^2} r \cos \theta \, dz \, r \, dr \, d\theta$$
Note that $$dx \, dy = r \, dr \, d\theta$$, so the $$dy \, dx$$ part becomes $$r \, dr \, d\theta$$.
6. **Integrate with respect to $$z$$:**
$$\int_{-5+r^2}^{5-r^2} r \cos \theta \, dz = r \cos \theta \times \left[(5 - r^2) - (-5 + r^2)\right] = r \cos \theta \times (10 - 2r^2)$$
7. **Substitute back and simplify:**
The integral becomes:
$$\int_0^{\pi/2} \int_0^2 r \cos \theta (10 - 2r^2) r \, dr \, d\theta = \int_0^{\pi/2} \int_0^2 r^2 \cos \theta (10 - 2r^2) \, dr \, d\theta$$
8. **Separate the integral:**
$$\int_0^{\pi/2} \cos \theta \, d\theta \times \int_0^2 r^2 (10 - 2r^2) \, dr$$
9. **Calculate the angular integral:**
$$\int_0^{\pi/2} \cos \theta \, d\theta = \sin \theta \Big|_0^{\pi/2} = 1 - 0 = 1$$
10. **Calculate the radial integral:**
$$\int_0^2 r^2 (10 - 2r^2) \, dr = \int_0^2 (10r^2 - 2r^4) \, dr = 10 \int_0^2 r^2 \, dr - 2 \int_0^2 r^4 \, dr$$
Calculate each:
- $$\int_0^2 r^2 \, dr = \frac{r^3}{3} \Big|_0^2 = \frac{8}{3}$$
- $$\int_0^2 r^4 \, dr = \frac{r^5}{5} \Big|_0^2 = \frac{32}{5}$$
So,
$$10 \times \frac{8}{3} - 2 \times \frac{32}{5} = \frac{80}{3} - \frac{64}{5} = \frac{400}{15} - \frac{192}{15} = \frac{208}{15}$$
11. **Final answer:**
$$\boxed{\frac{208}{15}}$$
This is the value of the triple integral after the change and conversion to polar coordinates.