It is equivalent to solving plane geometry problems: in an isosceles right-angled triangle, the hypotenuse = 20, and the right-angled side is found.

Let the right-angle side = x, x2+x2 = 20 2, 2x 2 = 400, x 2 = 200, x = 10, and the radical 2 is selected as A.

I remember later, in an isosceles right triangle, if the right side =a and the hypotenuse = root number 2 * a.

If hypotenuse =a, right angle =a/ root number 2.