MATH101 April 2011
• Q1 (a) • Q1 (b) • Q1 (c) • Q1 (d) • Q1 (e) • Q1 (f) • Q1 (g) • Q1 (h) • Q1 (i) • Q1 (j) • Q2 (a) • Q2 (b) • Q3 (a) • Q3 (b) • Q4 (a) • Q4 (b) • Q4 (c) • Q4 (d) • Q5 (a) • Q5 (b) • Q5 (c) • Q6 (a) • Q6 (b) • Q7 • Q8 •
Question 03 (b)

Find the centroid of the finite region in the plane bounded by the xaxis and the curve $\displaystyle {y=1+\sin x}$, for ${\frac {\pi }{2}}\leq x\leq {\frac {\pi }{2}}$.

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

Hint 1

The coordinates of the centroid are computed using the formulas
 $\displaystyle {{\bar {x}}={\frac {1}{A}}\int _{a}^{b}xf(x)dx}$
and
 $\displaystyle {{\bar {y}}={\frac {1}{A}}\int _{a}^{b}{\frac {1}{2}}(f(x))^{2}dx}$
where
 $A=\int _{a}^{b}f(x)\,dx$

Hint 2

 $\sin(x)^{2}={\frac {1\cos(2x)}{2}}$

Checking a solution serves two purposes: helping you if, after having used all the hints, you still are stuck on the problem; or if you have solved the problem and would like to check your work.
 If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work.
 If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies.
First we compute the area A of the shape. This is given by
 $\displaystyle {\begin{aligned}A&=\int _{\pi /2}^{\pi /2}1+\sin(x)dx\\&=x{\big }_{\pi /2}^{\pi /2}\cos {\big }_{\pi /2}^{\pi /2}\\&=\pi \cos(\pi /2)+\cos(\pi /2)\\&=\pi \end{aligned}}$
The coordinates of the centroid are computed using the formulas
 $\displaystyle {{\bar {x}}={\frac {1}{\pi }}\int _{\pi /2}^{\pi /2}x(1+\sin(x))dx}$
and
 $\displaystyle {{\bar {y}}={\frac {1}{\pi }}\int _{\pi /2}^{\pi /2}{\frac {1}{2}}(1+\sin(x))^{2}dx}$
We will compute ${\bar {x}}$ first.
 $\displaystyle {\begin{aligned}{\bar {x}}&={\frac {1}{\pi }}\int _{\pi /2}^{\pi /2}x+x\sin(x)dx\\&={\frac {1}{\pi }}\int _{\pi /2}^{\pi /2}xdx+{\frac {1}{\pi }}\int _{\pi /2}^{\pi /2}x\sin(x)dx\\&=0+{\frac {1}{\pi }}\int _{\pi /2}^{\pi /2}x\sin(x)dx\end{aligned}}$
The first integral is zero because the function is odd. For the second we can use integration by parts.
 $\displaystyle {\begin{aligned}\int x\sin(x)dx&=x\cos(x)+\int \cos(x)dx\\&=x\cos(x)+\sin(x)+C\end{aligned}}$
That means if we plug in the limits for the definite integral we get
 $\displaystyle {\begin{aligned}{\bar {x}}&=1/\pi (x\cos(x)+\sin(x)){\big }_{\pi /2}^{\pi /2}\\&=1/\pi (\sin(\pi /2)\sin(\pi /2))\\&=2/\pi \end{aligned}}$
Now we compute ${\bar {y}}$
 $\displaystyle {\begin{aligned}{\bar {y}}&={\frac {1}{\pi }}\int _{\pi /2}^{\pi /2}{\frac {1}{2}}(1+\sin(x))^{2}dx\\&={\frac {1}{2\pi }}\int _{\pi /2}^{\pi /2}1+2\sin(x)+\sin(x)^{2}dx\\&={\frac {1}{2\pi }}{\Big (}\int _{\pi /2}^{\pi /2}1dx+2\int _{\pi /2}^{\pi /2}\sin(x)dx+\int _{\pi /2}^{\pi /2}\sin(x)^{2}dx{\Big )}\\&={\frac {1}{2}}+{\frac {1}{2\pi }}\int _{\pi /2}^{\pi /2}\sin(x)^{2}dx\end{aligned}}$
To compute this we note $\sin(x)^{2}={\frac {1\cos(2x)}{2}}$
 $\displaystyle {\begin{aligned}\int \sin(x)^{2}dx&=\int {\frac {1\cos(2x)}{2}}dx\\&={\frac {x}{2}}{\frac {1}{4}}\sin(2x)+C\end{aligned}}$
Hence
 $\displaystyle {\begin{aligned}{\bar {y}}&={\frac {1}{2}}+{\frac {1}{2\pi }}({\frac {x}{2}}{\frac {1}{4}}\sin(2x))_{\pi /2}^{\pi /2}\\&={\frac {3}{4}}\end{aligned}}$
Thus the centroid is $({\frac {2}{\pi }},{\frac {3}{4}})$

Click here for similar questions
MER QGH flag, MER QGQ flag, MER QGS flag, MER RT flag, MER Tag Centroid