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# Let F = yi + zj + xzk. Evaluate Double integral F\cdot aS for each of the following regions W: a. x^2+y^2 (less than or equal to) z (less thanor equal to) 1 b. x^2+y^2 (less than or equal to) 1 and x (greater than or equal to) 0 # Let F = yi + zj + xzk. Evaluate Double integral F\cdot aS for each of the following regions W: a. x^2+y^2 (less than or equal to) z (less thanor equal to) 1 b. x^2+y^2 (less than or equal to) 1 and x (greater than or equal to) 0

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Other asked 2021-04-11
Let F = yi + zj + xzk. Evaluate
Double integral $$\displaystyle{F}\cdot{a}{S}$$ for each of the following regions W:
a. $$\displaystyle{x}^{{2}}+{y}^{{2}}$$ (less than or equal to) z (less thanor equal to) 1
b. $$\displaystyle{x}^{{2}}+{y}^{{2}}$$ (less than or equal to) 1 and x (greater than or equal to) 0

## Answers (1) 2021-04-13
div F=x
a) The given surface $$\displaystyle{x}^{{2}}+{y}^{{2}}\leq{z}\leq{1}$$ is the paraboloid $$\displaystyle{x}^{{2}}+{y}^{{2}}={z}$$ under the plane z=1. In cylinderical co-ordinates the region bounded by the surface is given by: $$\displaystyle{E}={\left[{\left({r},\theta,{z}\right)}:{0}\leq\theta\leq{2}\pi,{0}\leq{r}\leq{1},{r}^{{2}}\leq{z}\leq{1}\right]}$$
By Gauss theorem, $$\displaystyle\int\int_{{S}}{F}\cdot{d}{s}=\int\int\int_{{E}}\div{F}{d}{V}$$
$$\displaystyle={\int_{{0}}^{{{2}\pi}}}{\int_{{0}}^{{1}}}{\int_{{{r}^{{2}}}}^{{1}}}{r}{\cos{\theta}}{r}\ {\left.{d}{z}\right.}{d}{r}{d}\theta$$
$$\displaystyle={\int_{{0}}^{{{2}\pi}}}{\int_{{0}}^{{1}}}{r}^{{2}}{\cos{\theta}}{\left({1}-{r}^{{2}}\right)}{d}{r}{d}\theta$$
$$\displaystyle={\int_{{0}}^{{{2}\pi}}}{\cos{\theta}}{d}\theta{\int_{{0}}^{{1}}}{\left({r}^{{2}}-{r}^{{4}}\right)}{d}{r}$$
$$\displaystyle{{\left[{\sin{\theta}}\right]}_{{0}}^{{{2}\pi}}}{{\left[{\frac{{{r}^{{3}}}}{{{3}}}}-{\frac{{{r}^{{5}}}}{{{5}}}}\right]}_{{0}}^{{t}}}={0}$$
b) Note that here we have an extra condition $$\displaystyle{x}\geq{0}$$ which means that we have that portion of paraboloid under the plane which lies in front ofyz-plane. Then the region boundelby this surface is:
$$\displaystyle{E}={\left[{\left({r},\theta,{z}\right]}\right)}:-{\frac{{\pi}}{{{2}}}}\leq\theta\leq{\frac{{\pi}}{{{2}}}},{0}\leq{r}\leq{1},{r}^{{2}}\leq{z}\leq{1}{]}$$
$$\displaystyle\int\int{F}\cdot{d}{s}=\int\int\int_{{E}}\div{F}{d}{V}$$
Then
$$\displaystyle={\int_{{-{\frac{{\pi}}{{{2}}}}}}^{{\frac{{\pi}}{{{2}}}}}}{\int_{{0}}^{{1}}}{\int_{{{r}^{{2}}}}^{{1}}}{r}{\cos{\theta}}{r}{\left.{d}{z}\right.}{d}{r}{d}\theta$$
$$\displaystyle={{\left[{\sin{\theta}}\right]}_{{-{\frac{{\pi}}{{{2}}}}}}^{{{\frac{{\pi}}{{{2}}}}}}}{{\left[{\frac{{{r}^{{3}}}}{{{3}}}}-{\frac{{{r}^{{5}}}}{{{5}}}}\right]}_{{0}}^{{1}}}$$
$$\displaystyle={2}\times{\frac{{{2}}}{{{15}}}}={\frac{{{4}}}{{{15}}}}$$

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