Difference between revisions of "2010 AMC 8 Problems/Problem 18"
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Revision as of 00:51, 5 July 2013
Problem
A decorative window is made up of a rectangle with semicircles at either end. The ratio of 
to 
is 
. And is 30 inches. What is the ratio of the area of the rectangle to the combined area of the semicircle.
![[asy] import graph; size(5cm); real lsf=0; pen dps=linewidth(0.7)+fontsize(8); defaultpen(dps); pen ds=black; real xmin=-4.27,xmax=14.73,ymin=-3.22,ymax=6.8; draw((0,4)--(0,0)); draw((0,0)--(2.5,0)); draw((2.5,0)--(2.5,4)); draw((2.5,4)--(0,4)); draw(shift((1.25,4))*xscale(1.25)*yscale(1.25)*arc((0,0),1,0,180)); draw(shift((1.25,0))*xscale(1.25)*yscale(1.25)*arc((0,0),1,-180,0)); dot((0,0),ds); label("$A$",(-0.26,-0.23),NE*lsf); dot((2.5,0),ds); label("$B$",(2.61,-0.26),NE*lsf); dot((0,4),ds); label("$D$",(-0.26,4.02),NE*lsf); dot((2.5,4),ds); label("$C$",(2.64,3.98),NE*lsf); clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle);[/asy]](http://latex.artofproblemsolving.com/1/b/d/1bd7bb101d1f617dcc2d568edfde1a17ef3345b4.png)
Solution
We can set a proportion:
![\[\dfrac{AD}{AB}=\dfrac{3}{2}\]](http://latex.artofproblemsolving.com/9/d/c/9dc8ec1f9de87f93331eca33e68b9a9bfb4fe1be.png)
We substitute with 30 and solve for AD.
![\[\dfrac{AD}{30}=\dfrac{3}{2}\]](http://latex.artofproblemsolving.com/b/d/c/bdc8fcd7c68f8cda8f0d49383feb93a5f8415894.png)
![\[AD=45\]](http://latex.artofproblemsolving.com/f/1/e/f1e60700abae8840538f90cd4b41361fd259e31e.png)
We calculate the combined area of semicircle by putting together semicircle and 
to get a circle with radius 
. Thus, the area is 
. The area of the rectangle is 
. We calculate the ratio:
![\[\dfrac{1350}{225\pi}=\dfrac{6}{\pi}\Rightarrow\boxed{\textbf{(C)}\ 6:\pi}\]](http://latex.artofproblemsolving.com/e/2/4/e2419c749922191e5f080078e22383b0ce4f17ae.png)
See Also
| 2010 AMC 8 (Problems • Answer Key • Resources) | ||
| Preceded by Problem 17 |
Followed by Problem 19 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
| All AJHSME/AMC 8 Problems and Solutions | ||
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. 
