Four Squares in Circle
6 May 2020
Puzzle

Area of each square is 16. What is the area of the circle?

Source

Heard from an IIT Delhi classmate in April 2020. Don't remember who posted this puzzle in a group.

Solution

This problem can be solved in many different ways. For example, we may trigonometry or coordinate geometry (given three points in a circle, find its radius). However, these approaches don't unearth this elegant underlying structure of the problem: 6^2 + 7^2 = 9^2 + 2^2 = 85, which is a visual delight!

How may we arrive at the visually delightful solution below? We may reflect the four squares along the a vertical line (parallel to y-axis) passing through the center of the circle. Clearly, the horizontal lines of the grid overlap. We have to establish that the vertical lines also overlap. Is there a simple argument for that?

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