Question

http://imageshack.us/photo/my-images/820/tritr.png/
How do I know if A is and right angle?

In the problem, y = 7 and R = 637. And I have to find X.
How would I do this?

Answer 1

R is the radius of the circle.

Answer 2

since A is an angle on the circumference of the circle,also an angle of the semicircle if we further consider we can take it as 90