The answer depends on the track. They are not all the same shape.
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It is the same as the radius of the whole circle.
The radius of a semi-circle is the same as the radius of the circle that was cut in half. The radius of a circle is the distance from the center of the circle to any point on the circle. That's the same as 1/2 of the diameter. It's easy to measure it on a semi-circle, because you don't even need to determine where the center is. The diameter is just the length of the flat side, and the radius is half of that.
Just the one and it is its radius when at right angles to the semi-circle's centre
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find the circumferance as if it was a whole circle but when you get the answer divide it by 2