2005 PMWC Problems/Problem I4
Problem
The larger circle has radius 12 cm. Each of the six identical smaller circles touches its two neighbours and the larger circle. What is the radius of the smaller circle?
![[asy] unitsize(0.5cm); draw((0,3)..(3,0)..(0,-3)..(-3,0)..cycle); for (int i=0;i<6;i=i+1){ draw(dir(60*i)..3*dir(60*i)..cycle); } [/asy]](http://latex.artofproblemsolving.com/6/e/3/6e3d06c0755130a0ee8410321e24b4f90f9aabd9.png)
Solution
![[asy] unitsize(0.5cm); draw((0,3)..(3,0)..(0,-3)..(-3,0)..cycle); for (int i=0;i<6;i=i+1){ draw(dir(60*i)..3*dir(60*i)..cycle); } draw((0,1)..(0,-1)..cycle,rgb(1,0,0)); [/asy]](http://latex.artofproblemsolving.com/e/8/b/e8b52e4bb60df8603b471083d289883445019a63.png)
Adding a circle in the middle makes the solution obvious. Since the radius of the big circle is 
cm, then the diameter of the big circle is 
cm. But this is also 
of the little circle's diameter! Therefore, the diameter of the little circle is 
cm and the radius is therefore 
.
See also
| 2005 PMWC (Problems) | ||
| Preceded by Problem I3 |
Followed by Problem I5 | |
| I: 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 T: 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 | ||
