To draw an arc by tangent, use the Arc by Tangent tool. Select an edge or endpoint to start a tangent arc. An arc is then rubber banded. You control the radius and angular extent of the arc with the cursor. Which way the arc curves from your initial selection depends on your cursor position. If you imagine a tangent line extending out from this first location, which side of that line your cursor is on effects the direction the arc curves. Also, if you want the second end of the arc to be tangent to another curve, watch for the 'T' to be displayed, if you have the Auto Constraints Cursor turned on, when selecting the second curve. You can use the right mouse button Reverse option to reverse the initial direction of the arc. If you want it to be tangent to the end of an edge, be sure to select its endpoint instead of the edge itself. If you select a center point, Construction Point, or nothing at all, a 180° arc will be created.

Note that if you are selecting a point where there is more than one possible endpoint, and not getting the direction you want, then instead select the edge as close as possible to its end. That way, Auto-Constraints should still snap the arc to the edge endpoint.