Question

A circle has its center at the origin, and (5, -12) is a point on the circle. How long is the radius of the circle?

Answer 1

Hello here is a solution :
 the radius of the circle is OA        A(5;-12) and O (0;0)
OA=   square root ((5-0)² + (-12-0)²) 
     =   square root (25+144) 
     =   square root(169)
 OA = 13

Answer 2

13, because I said so.