Three triangles and three circles

Geometry Level 3

A B AB is a straight line of length 6 6 . Point C C is on A B AB such that B C = 2 BC=2 and C A = 4 CA=4 . Three equilateral triangles are drawn as shown. If L , M L,M and N N are the centers of the circles, find the area of M L N \triangle MLN . If your answer can be expressed as a b b \dfrac{a}{b}\sqrt{b} where a a and b b are positive coprime intergers and b b is square free, find a b ab .


The answer is 21.

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1 solution

In triangle L O A LOA ,

cos 30 = 2 L A \cos 30=\dfrac{2}{LA} \implies 3 2 = 2 L A \dfrac{\sqrt{3}}{2}=\dfrac{2}{LA} \implies L A = 4 3 LA=\dfrac{4}{\sqrt{3}}

In triangle A M P AMP ,

cos 30 = 3 M A \cos 30=\dfrac{3}{MA} \implies 3 2 = 3 M A \dfrac{\sqrt{3}}{2}=\dfrac{3}{MA} \implies M A = 6 3 MA=\dfrac{6}{\sqrt{3}}

Apply cosine law in triangle A L M ALM ,

( L M ) 2 = ( L A ) 2 + ( M A ) 2 2 ( L A ) ( M A ) ( cos 60 ) = ( 4 3 ) 2 + ( 6 3 ) 2 2 ( 4 3 ) ( 6 3 ) ( 1 2 ) = 16 3 + 12 8 = 28 3 (LM)^2=(LA)^2+(MA)^2-2(LA)(MA)(\cos 60)=\left(\dfrac{4}{\sqrt{3}}\right)^2+\left(\dfrac{6}{\sqrt{3}}\right)^2-2\left(\dfrac{4}{\sqrt{3}}\right)\left(\dfrac{6}{\sqrt{3}}\right)\left(\dfrac{1}{2}\right)=\dfrac{16}{3}+12-8=\dfrac{28}{3}

Since triangle M N L MNL is equilateral (the proof is left to the solver), the area is

A = 1 2 ( L M ) 2 ( sin 60 ) = 1 2 ( 28 3 ) ( 3 2 ) = 7 3 3 A=\dfrac{1}{2}(LM)^2(\sin 60)=\dfrac{1}{2}\left(\dfrac{28}{3}\right)\left(\dfrac{\sqrt{3}}{2}\right)=\dfrac{7}{3}\sqrt{3}

The desired answer is a b = 7 ( 3 ) = ab=7(3)= 21 \boxed{21}

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