Two adjacent vertices of the square (ABCD) lie on the diameter of that circle, while the other two vertices lie on the circumference of the circle.
What is the length of a diagonal of this square (DB)?
Let's designate the length of OC as x. then:
x^2 + (2x)^2 = 36
5 * x^2 = 36
x^2 = 36/5
x = 6/SQRT(5)
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