2006 AIME A Problems/Problem 10
Problem
Eight circles of diameter 1 are packed in the first quadrant of the coordinte plane as shown. Let region 
be the union of the eight circular regions. Line 
with slope 3, divides into two regions of equal area. Line 
's equation can be expressed in the form 
where 
and 
are positive integers whose greatest common divisor is 1. Find 
Solution
Assume that if unit squares are drawn circumscribing the circles, then the line will divide the area of the concave hexagonal region of the squares equally (proof needed). Denote the intersection of the line and the x-axis as 
.
The line divides the region into 2 sections. The left piece is a trapezoid, with its area 
. The right piece is the addition of a trapezoid and a rectangle, and the areas are 
and 
, totaling 
. Since we want the two regions to be equal, we find that 
, so 
.
We have that 
is a point on the line of slope 3, so 
and 
. In y-intercept form, the equation of the line is 
, and in the form for the answer, the line’s equation is 
. Thus, our answer is 
.
