Question

Let c,r be constants, and D={(x,y,z):x^2+y^2+z^2≤r^2}. The answer to ∭D cdV


is

Select one:

a. (π^2cr^3)/3



b. (4πcr^3)/3


c. 4πcr^3


d. (πcr^4)/3



e. (4πcr^2)/2


f. (4r^3)/3



g. (πcr^3)/3

Answer 1

$\text{Since we have}\\ x^2+y^2+z^2\leq r^2,\\ we \ get\\ \rho=0 \rightarrow r\\ \phi=0 \rightarrow \pi \ \text{and}\\ \theta=0 \rightarrow 2 \pi\\ \text{So, we obtain}\\ \iiint\limits_{D} c d V=\int_{0}^{2 \pi} \int_{0}^{\pi} \int_{0}^{r} c \rho^{2} \sin (\phi) d \rho d \phi d \theta\\ =-\left.\left.\left.c \frac{\rho^{3}}{3}\right|_{0} ^{r} \cos (\phi)\right|_{0} ^{\pi} \theta\right|_{0} ^{2 \pi}\\ =-c \frac{r^{3}}{3}(\cos (\pi)-\cos (0)(2 \pi-0))\\ =-c \frac{r^{3}}{3}(-2)(2 \pi)\\ =\frac{4}{3} \pi c r^{3}\\ \text{So, the correct answer is }\ \ (b)$