radius of an arc created with UIbezier

radius of an arc created with UIbezier

I created part of a circle with UIBezierPath:
UIBezierPath * aPath = [UIBezierPath bezierPathWithArcCenter: arc
radius: 75
startAngle: 0
endAngle:
DEGREES_TO_RADIANS
(135)
clockwise: YES];
[[UIColor yellowColor] setStroke];
[aPath stroke];
But if you then add it to the center of the circle radius, the radius does
not reach the edge of the circle, arc so to speak .. why? What can I do?
In this way the calculations are unnecessary to the radius