Say, an arc with radius r centered at o, from angle a to angle b.
I don't see how to use Arc tool in Adobe Illustrator this way. What's the best way to draw it?
AI sucks for this. Illustrator doesn't draw "real" circles (ie., using centerpoint and radius) either, it makes an Ellipse using bezier curves.
Your best bet is to make a Ellipse with equal width & height (ie., a circle) and use 2 Line Segment shapes to intersect the Ellipse. It takes 3-4 steps instead of 1, but it will allow you to use specific dimensions.