Open Middle®
Directions: Using the digits 1 to 9, as many times as you want, place a digit in each box to make the biggest circle possible.
Hint
Where is it better to put the larger values?
Answer
To make the circle bigger, the radius needs to be as large as possible. So, putting a 9 there makes it the biggest. The coefficients of x and y need to be as small as possible so they both need to be 1.
Source: Robert Kaplinsky