A circle with radius r has a tangent line at a point P on the circle. The distance of the tangent from point P to the external point Q is 10 cm. The distance from the center of the circle to point Q is 13 cm. Find the radius of the circle.
√69
The radius drawn to the point of tangency is perpendicular to the tangent line, so triangle OPQ is right-angled at P. By the Pythagorean theorem, \(OQ^2 = OP^2 + PQ^2\), giving \(13^2 = r^2 + 10^2\), so \(r^2 = 169 - 100 = 69\), and \(r = \sqrt{69}\).
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