In , . Three tangent circles are to be inscribed in the triangle as shown in the figure above. If is the sum of their radii, enter
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Let the radii of the three circles be r A , r B , r C . Label the points of tangency along B C as P , Q as below:
Since the centre of the circle nearest B lies on the bisector of ∠ B , we have B P = r B cot 2 B . Similarly, C Q = r C cot 2 C .
By Pythagoras, P Q 2 = ( r B + r C ) 2 − ( r B − r C ) 2 , so that P Q = 2 r B r C .
Putting all these together, B C = r B cot 2 B + 2 r B r C + r C cot 2 C
We can use the same approach on the other two sides to find C A = r C cot 2 C + 2 r C r A + r A cot 2 A and A B = r A cot 2 A + 2 r A r B + r B cot 2 B
Since we know the sides of Δ A B C , 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 . 3 3 5 9 6 … , r B = 1 . 8 1 8 2 7 … , r C = 2 . 1 2 9 3 7 …
giving an answer of 5 2 8 3 6 0 .