Let's look at the first and second circles. The radius of the first is 1. The radius of the second is the distance between the origin and any of its points. One of its points is (.5,.5), where the circle is tangent to the line that goes from (1,0) to (0,1). The distance from the origin to (.5,.5) is 1/sqrt(2).
Jetru wrote:I didn't get the second question. 1+2+3 <20
That was the first question. What he meant is that no matter how you arrange the numbers from 1 to 12 on a clock, there is always going to be a group of three consecutive numbers whose sum is at least 20. Actually, the result can be improved to 21.