Try this beautiful problem from Geometry based on Circumscribed Circle
Problem on Circumscribed Circle – AMC-10A, 2003- Problem 17
The number of inches in the perimeter of an equilateral triangle equals the number of square inches in the area of its circumscribed circle. What is the radius, in inches, of the circle?
- \(\frac{5\sqrt3}{\pi}\)
- \(\frac{3\sqrt3}{\pi}\)
- \(\frac{3\sqrt3}{2\pi}\)
Key Concepts
Geometry
Triangle
Circle
Check the Answer
Answer: \(\frac{3\sqrt3}{\pi}\)
AMC-10A (2003) Problem 17
Pre College Mathematics
Try with Hints

Let ABC is a equilateral triangle which is inscribed in a circle. with center \(O\). and also given that perimeter of an equilateral triangle equals the number of square inches in the area of its circumscribed circle.so for find out the peremeter of Triangle we assume that the side length of the triangle be \(x\) and the radius of the circle be \(r\). then the side of an inscribed equilateral triangle is \(r\sqrt{3}\)=\(x\)
Can you now finish the problem ……….

The perimeter of the triangle is=\(3x\)=\(3r\sqrt{3}\) and Area of the circle=\(\pi r^2\)
Now The perimeter of the triangle=The Area of the circle
Therefore , \(3x\)=\(3r\sqrt{3}\)=\(\pi r^2\)
can you finish the problem……..
Now \(3x\)=\(3r\sqrt{3}\)=\(\pi r^2\) \(\Rightarrow {\pi r}=3\sqrt 3\) \(\Rightarrow r=\frac{3\sqrt3}{\pi}\)
Other useful links
- https://www.cheenta.com/geometry-based-on-triangle-prmo-2018-problem-6
- https://www.youtube.com/watch?v=7AlfBAPWEMg