Multivariable Calculus : Parameterization & Surface Integrals

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Example Questions

Example Question #1 : Triple Integration Of Surface

Evaluate \(\displaystyle \int \int \int_D y \ dV\), where \(\displaystyle D\) is the region below the plane \(\displaystyle z=x+1\) , above the \(\displaystyle xy\) plane and between the cylinders \(\displaystyle x^2+y^2=1\), and \(\displaystyle x^2+y^2=9\).

Possible Answers:

\(\displaystyle \frac{\pi}{2}\)

\(\displaystyle \pi\)

\(\displaystyle 2 \pi\)

\(\displaystyle \frac{\pi}{4}\)

\(\displaystyle 0\)

Correct answer:

\(\displaystyle 0\)

Explanation:

We need to figure out our boundaries for our integral.

We need to convert everything into cylindrical coordinates. Remeber we are above the \(\displaystyle xy\) plane, this means we are above \(\displaystyle z=0\).

\(\displaystyle 0\leq z \leq x+1 \rightarrow 0\leq z \leq r\cos(\theta)+1\)

The region \(\displaystyle D\) is between two circles \(\displaystyle x^2+y^2=1\), and \(\displaystyle x^2+y^2=9\).

This means that 

\(\displaystyle 0 \leq \theta \leq 2\pi\)

\(\displaystyle 1\leq r \leq 3\)

\(\displaystyle \int \int \int_D y \ dV=\int_{0}^{2\pi} \int_{1}^{3} \int_{0}^{r\cos(\theta)+1} r\sin(\theta) r\ dz \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \int_{1}^{3} \int_{0}^{r\cos(\theta)+1} r^2\sin(\theta) \ dz \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \int_{1}^{3} zr^2\sin(\theta) \Big|_{0}^{r\cos(\theta)+1} \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \int_{1}^{3} (r\cos(\theta)+1)r^2\sin(\theta)-0 \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \int_{1}^{3} r^3\sin(\theta)\cos(\theta)+r^2\sin(\theta) \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \frac{1}{4}r^4\sin(\theta)\cos(\theta)+\frac{1}{3}r^3\sin(\theta) \Big|_{1}^{3}\ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} (\frac{1}{4}(3)^4\sin(\theta)\cos(\theta)+\frac{1}{3}(3)^3\sin(\theta))-(\frac{1}{4}(1)^4\sin(\theta)\cos(\theta)+\frac{1}{3}(1)^3\sin(\theta)) \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} (\frac{81}{4}\sin(\theta)\cos(\theta)+9\sin(\theta))-(\frac{1}{4}\sin(\theta)\cos(\theta)+\frac{1}{3}\sin(\theta)) \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} 20\sin(\theta)\cos(\theta)+\frac{26}{3}\sin(\theta) \ d\theta\)

\(\displaystyle = -\frac{1}{2}\cdot20\cos^2(\theta)-\frac{26}{3}\cos(\theta)\Big|_{0}^{2\pi}\)

\(\displaystyle = -\frac{1}{2}\cdot20\cos^2(2\pi)-\frac{26}{3}\cos(2\pi)-(-\frac{1}{2}\cdot20\cos^2(0)-\frac{26}{3}\cos(0))\)

\(\displaystyle = -\frac{1}{2}\cdot20(1)-\frac{26}{3}(1)-(-\frac{1}{2}\cdot20(1)-\frac{26}{3}(1))\)

\(\displaystyle = -10-\frac{26}{3}-(-10-\frac{26}{3})=0\)

 

Example Question #1 : Parameterization & Surface Integrals

Evaluate \(\displaystyle \int \int \int_D y \ dV\), where \(\displaystyle D\) is the region below the plane \(\displaystyle z=x+1\) , above the \(\displaystyle xy\) plane and between the cylinders \(\displaystyle x^2+y^2=1\), and \(\displaystyle x^2+y^2=9\).

Possible Answers:

\(\displaystyle 0\)

\(\displaystyle 2 \pi\)

\(\displaystyle \frac{\pi}{2}\)

\(\displaystyle \frac{\pi}{4}\)

\(\displaystyle \pi\)

Correct answer:

\(\displaystyle 0\)

Explanation:

We need to figure out our boundaries for our integral.

We need to convert everything into cylindrical coordinates. Remeber we are above the \(\displaystyle xy\) plane, this means we are above \(\displaystyle z=0\).

\(\displaystyle 0\leq z \leq x+1 \rightarrow 0\leq z \leq r\cos(\theta)+1\)

The region \(\displaystyle D\) is between two circles \(\displaystyle x^2+y^2=1\), and \(\displaystyle x^2+y^2=9\).

This means that 

\(\displaystyle 0 \leq \theta \leq 2\pi\)

\(\displaystyle 1\leq r \leq 3\)

\(\displaystyle \int \int \int_D y \ dV=\int_{0}^{2\pi} \int_{1}^{3} \int_{0}^{r\cos(\theta)+1} r\sin(\theta) r\ dz \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \int_{1}^{3} \int_{0}^{r\cos(\theta)+1} r^2\sin(\theta) \ dz \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \int_{1}^{3} zr^2\sin(\theta) \Big|_{0}^{r\cos(\theta)+1} \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \int_{1}^{3} (r\cos(\theta)+1)r^2\sin(\theta)-0 \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \int_{1}^{3} r^3\sin(\theta)\cos(\theta)+r^2\sin(\theta) \ dr \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} \frac{1}{4}r^4\sin(\theta)\cos(\theta)+\frac{1}{3}r^3\sin(\theta) \Big|_{1}^{3}\ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} (\frac{1}{4}(3)^4\sin(\theta)\cos(\theta)+\frac{1}{3}(3)^3\sin(\theta))-(\frac{1}{4}(1)^4\sin(\theta)\cos(\theta)+\frac{1}{3}(1)^3\sin(\theta)) \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} (\frac{81}{4}\sin(\theta)\cos(\theta)+9\sin(\theta))-(\frac{1}{4}\sin(\theta)\cos(\theta)+\frac{1}{3}\sin(\theta)) \ d\theta\)

\(\displaystyle =\int_{0}^{2\pi} 20\sin(\theta)\cos(\theta)+\frac{26}{3}\sin(\theta) \ d\theta\)

\(\displaystyle = -\frac{1}{2}\cdot20\cos^2(\theta)-\frac{26}{3}\cos(\theta)\Big|_{0}^{2\pi}\)

\(\displaystyle = -\frac{1}{2}\cdot20\cos^2(2\pi)-\frac{26}{3}\cos(2\pi)-(-\frac{1}{2}\cdot20\cos^2(0)-\frac{26}{3}\cos(0))\)

\(\displaystyle = -\frac{1}{2}\cdot20(1)-\frac{26}{3}(1)-(-\frac{1}{2}\cdot20(1)-\frac{26}{3}(1))\)

\(\displaystyle = -10-\frac{26}{3}-(-10-\frac{26}{3})=0\)

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