In triangle ABC, AC = 15, BC = 5√7, angle C is 90 °. Find the radius of the circumscribed circle of this triangle

The triangle ABC is rectangular, then by the Pythagorean theorem:
AB2 = AC2 + BC2
AB ^ 2 = 15 ^ 2 + (5√7) ^ 2
AB ^ 2 = 225 + 25 * 7
AB ^ 2 = 400
AB = 20
Since the triangle ABC is right-angled, this means that the center of the circle is in the middle of the hypotenuse (by the theorem on the circumscribed circle).
Then R = AB / 2 = 20/2 = 10
Answer: R = 10