You could also set up a Short Cut Key by going to View →ToolBars→Customize and choosing the Keyboard tab. Find the tool Tangent Arc and add in a Short cut.
You could even add a Mouse Gesture. Also found under View →ToolBars→ Customize this time you choose the Mouse Gesture tab. Find Tangent Arc and choose the mouse movement you would like.
These are all great but there is a better way! SolidWorks has provided the “A” key! All you have to do is start drawing your sketch using the line tool.
Whenever you want an arc that is tangent to the line you just drew, press the “A” key.
You can always get back to a line by pressing the “A” key once again.
Keep in mind that the placement of your curser when you press the “A” key will decided the direction of the Tangent.
Now you can draw lines and Tangent arcs without ever dropping a tool!