Perimeter Ratios with Angle Bisector

Geometry Level pending

In the diagram below, A B C \triangle{ABC} has right B \angle{B} . Because of the fact that point D D lies on B C \overline{BC} such that A D \overline{AD} bisects B A C \angle{BAC} and the ratio C D : D B = 2 : 1 \overline{CD}:\overline{DB}=2:1 , the ratio of the perimeter of A D C \triangle{ADC} to the perimeter of A B D \triangle{ABD} can be written in the form a + b c \frac{a+\sqrt{b}}{c} , where a a and c c are prime integers and b b is square-free. What is a + b + c a+b+c ?


The answer is 9.

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1 solution

Lys Lol
Oct 14, 2018

We can see that triangle DBA is similar to triangle ABC. So angle ADB is same as angle CAB. Then let one of the bisected angles be x. A equation can be formed: 90-x=2x. x will be 30 degrees. Then using trigonometry to solve for the length of each side. We will have the ratio of the perimeters in the form of (4+sqrt12)/(3+sqrt3). Next, rationalize the denominator to get (3+sqrt3)/3. Hence, a+b+c=3+3+3=9.

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