Dividing triangle

Geometry Level 3

In the given A B C \triangle ABC , the red areas are equal, A D = 5 D B AD = 5 DB and A E E B = a b + b c \dfrac{AE}{EB} = \dfrac{a \sqrt b + b}{c} , where a a , b b and c c are coprime positive integers. Find a + b + c a+b+c .


The answer is 42.

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1 solution

From the left figure, the red triangle is similar to A B C \triangle ABC . Therefore, the ratio of their areas A r e d A = A D 2 ( A D + D B ) 2 = 5 2 ( 5 + 1 ) 2 = 25 36 \dfrac {A_{\color{#D61F06}red}}{A_\triangle} = \dfrac {AD^2}{(AD+DB)^2} = \dfrac {5^2}{(5+1)^2} = \dfrac {25}{36} . A r e d = 25 36 A \implies A_{\color{#D61F06}red} = \dfrac {25}{36}A_\triangle , A b l u e = A A r e d = 11 36 A \implies A_{\color{#3D99F6}blue} = A_\triangle - A_{\color{#D61F06}red} = \dfrac {11}{36}A_\triangle .

From the right figure,

A b l u e A = 11 36 A E 2 ( A E + E B ) 2 = 11 36 Taking square root both sides A E A E + E B = 11 6 Divide up and down of LHS by E B A E E B A E E B + 1 = 11 6 A E E B = 11 6 11 = 11 ( 6 + 11 ) ( 6 11 ) ( 6 + 11 ) = 6 11 + 11 25 \begin{aligned} \frac {A_{\color{#3D99F6}blue}}{A_\triangle} & = \frac {11}{36} \\ \implies \frac {AE^2}{(AE+EB)^2} & = \frac {11}{36} & \small \color{#3D99F6} \text{Taking square root both sides} \\ \frac {AE}{AE+EB} & = \frac {\sqrt {11}}6 & \small \color{#3D99F6} \text{Divide up and down of LHS by }EB \\ \frac {\frac {AE}{EB}}{\frac {AE}{EB}+1} & = \frac {\sqrt {11}}6 \\ \implies \frac {AE}{EB} & = \frac {\sqrt{11}}{6-\sqrt{11}} \\ & = \frac {\sqrt{11}(6+\sqrt{11})}{(6-\sqrt{11})(6+\sqrt{11})} \\ & = \frac {6\sqrt{11}+11}{25} \end{aligned}

Therefore, a + b + c = 6 + 11 + 25 = 42 a+b+c = 6+11+25 = \boxed{42} .

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