Heptaparaparshinokh

Geometry Level pending

A B C ABC is a 13 13 - 14 14 - 15 15 triangle. E D A B ED \parallel AB and G D E B GD \perp EB . If the three small circles are congruent and the radius of the large circle is p q \dfrac pq , where p p and q q are coprime positive integers, submit p + q p+q .


The answer is 89.

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2 solutions

Chew-Seong Cheong
May 15, 2021

Let E B EB and D G DG intersects at H H . We note that the four triangles E D H EDH , D B H DBH , H B G HBG , and E H G EHG are congruent and E G D B EG \parallel DB . Also that E D C \triangle EDC , A G E \triangle AGE , and A B C \triangle ABC are similar.

Let D B = E G = x DB = EG = x . Then E G A G = B C A B \dfrac {EG}{AG} = \dfrac {BC}{AB} , x 14 x = 15 14 x = 14 × 15 29 \implies \dfrac x{14-x} = \dfrac {15}{14} \implies x = \dfrac {14 \times 15}{29} . Let the radius of the large circle be r r and the inradius of A B C \triangle ABC be R R . Then

r R = C D B C = 15 x 15 = 15 29 r = 15 29 R = 15 29 × A s = 15 29 × 1 2 × 14 × 12 1 2 ( 13 + 14 + 15 ) = 60 29 \begin{aligned} \frac rR & = \frac {CD}{BC} = \frac {15-x}{15} = \frac {15}{29} \\ \implies r & = \frac {15}{29} R = \frac {15}{29} \times \frac As = \frac {15}{29} \times \frac {\frac 12 \times 14 \times 12}{\frac 12 (13+14+15)} = \frac {60}{29} \end{aligned}

Therefore p + q = 60 + 29 = 89 p+q = 60+29 = \boxed{89} .

Saya Suka
May 14, 2021

The congruence of the smaller circles hinted at the rhombic overall shape of the right triangles containing them, and we can use the property of rhombus' angle bisection here.

3/5 = 2(cos² €) – 1
cos € = 2/√5

Let's have the small RTs with the little incircles having legs x and 2x and hypothenuse √5x. With that, the triangle containing the larger circle would have 2 of its edges as √5x and (15 – √5x) that are relatively comparable to the original triangle's edges of 14 and 15 respectively.

√5x / (15 – √5x) = 14 / 15
x = 42√5 / 29

Ratio of triangles (smaller / original)
= [ (42√5 / 29) × √5 ] / 14
= 15 / 29

Original triangle's inradius
= (Double area) / Perimeter
= 14 × 12 / (13 + 14 + 15)
= 4

Inradius
= (Ratio) × (Original's)
= 60 / 29

Answer = 60 + 29 = 89

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