is a straight line of length . Point is on such that and . Three equilateral triangles are drawn as shown. If and are the centers of the circles, find the area of . If your answer can be expressed as where and are positive coprime intergers and is square free, find .
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In triangle L O A ,
cos 3 0 = L A 2 ⟹ 2 3 = L A 2 ⟹ L A = 3 4
In triangle A M P ,
cos 3 0 = M A 3 ⟹ 2 3 = M A 3 ⟹ M A = 3 6
Apply cosine law in triangle A L M ,
( L M ) 2 = ( L A ) 2 + ( M A ) 2 − 2 ( L A ) ( M A ) ( cos 6 0 ) = ( 3 4 ) 2 + ( 3 6 ) 2 − 2 ( 3 4 ) ( 3 6 ) ( 2 1 ) = 3 1 6 + 1 2 − 8 = 3 2 8
Since triangle M N L is equilateral (the proof is left to the solver), the area is
A = 2 1 ( L M ) 2 ( sin 6 0 ) = 2 1 ( 3 2 8 ) ( 2 3 ) = 3 7 3
The desired answer is a b = 7 ( 3 ) = 2 1