The circumference of the circle is 30π. (π * diameter). The length of the arc is 1/4 the length of the diameter because 4(15π/2)=30π. Now it is easy to see that the angle AOB, where O is the center of the circle, is 90˚. Now we just need to use the pythagorean theorem to solve a triangle with 2 legs of length 30 to get 30sqrt2.
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