Fifteen unit circles are inscribed in an equilateral triangle in such a way that each circle is externally tangent to its neighbors. What is the area of the triangle?
Not as hard as it seems
Call the lower left vertex of the triangle, B
Let the center of the bottom left circle be A
Let the radius of this circle intersect the base of the triangle at C
So.....triangle ABC is a 30 -60 -90 right triangle
And BC = radius of the circle * sqrt (3) = 1 * √ (3) = √ (3)
So....using symmetry...the side of the triangle =
2BC + 8 r =
2sqrt (3) + 8 r =
2sqrt (3) + 8 =
√12 + 8
So....the area of the triangle =
(1/2) ( √12 + 8)^2 * (√3/2) =
(√3/4) ( 12 + 16√12 + 64) =
(√3/4) ( 76 + 16*2√3) =
√3 ( 19 + 8√3) =
19√3 + 24 units^2 ≈ 56.9 units^2