Three tangent circles in a triangle

Geometry Level pending

In A B C \triangle ABC , A B = 8 , B C = 15 , A C = 12 \overline{AB} = 8, \overline{BC} = 15, \overline{AC} = 12 . Three tangent circles are to be inscribed in the triangle as shown in the figure above. If S S is the sum of their radii, enter 1 0 5 S \lfloor 10^5 S \rfloor


The answer is 528360.

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1 solution

Chris Lewis
Jan 26, 2021

Let the radii of the three circles be r A , r B , r C r_A,r_B,r_C . Label the points of tangency along B C BC as P , Q P,Q as below:

Since the centre of the circle nearest B B lies on the bisector of B \angle B , we have B P = r B cot B 2 BP=r_B \cot \frac{B}{2} . Similarly, C Q = r C cot C 2 CQ=r_C \cot \frac{C}{2} .

By Pythagoras, P Q 2 = ( r B + r C ) 2 ( r B r C ) 2 PQ^2 = \left(r_B+r_C \right)^2 - \left(r_B-r_C \right)^2 , so that P Q = 2 r B r C PQ=2\sqrt{r_B r_C} .

Putting all these together, B C = r B cot B 2 + 2 r B r C + r C cot C 2 BC=r_B \cot \frac{B}{2} + 2\sqrt{r_B r_C} + r_C \cot \frac{C}{2}

We can use the same approach on the other two sides to find C A = r C cot C 2 + 2 r C r A + r A cot A 2 CA=r_C \cot \frac{C}{2} + 2\sqrt{r_C r_A} + r_A \cot \frac{A}{2} and A B = r A cot A 2 + 2 r A r B + r B cot B 2 AB=r_A \cot \frac{A}{2} + 2\sqrt{r_A r_B} + r_B \cot \frac{B}{2}

Since we know the sides of Δ A B C \Delta ABC , we can work out the required angles as well. I didn't find a nice algebraic solution so plugged the equations into Wolfram|Alpha to find r A = 1.33596 , r B = 1.81827 , r C = 2.12937 r_A=1.33596\ldots,\;\;\;\;r_B=1.81827\ldots,\;\;\;\;r_C=2.12937\ldots

giving an answer of 528360 \boxed{528360} .

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