Try this beautiful problem from Geometry based on the radius and tangent of a circle.

SMO 2013 – Geometry (Problem 25)

As shown in the figure below ,circles $C_1 $and$ C_2$ of radius 360 are tangent to each other , and both tangent to the straight line l.if the circle$ C_3$ is tangent to $C_1$ ,$C_2$ and l ,and circle$ C_4 $is tangent to$ C_1$,$C_3$ and l ,find the radius of$ C_4$

radius of a circle

  • 30
  • 35
  • 40

Key Concepts


Pythagoras theorm

Distance Formula

Check the Answer

But try the problem first…


Suggested Reading

SMO -Math Olympiad-2013

Pre College Mathematics

Try with Hints

First hint

Let R be the radius of $C_3$

$C_2E$ =360-R



Using pythagoras theorm ….

$ (360-R)^2+360^2=(360+R)^2$

i.e R=90

Can you now finish the problem ……….

Second Hint

Let the radius of$ C_4$ be r

then use the distacce formula and tangent property……..

can you finish the problem……..

Final Step

Let r be the radius of $C_4$ (small triangle).



i.e r=40.

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