The Tangent Line tab of the Settings dialog box allows you to choose between several different input methods for specifying where to draw tangent lines:
Select initial point/curve with mouse This option is checked by default. When this box is checked, choosing Draw Tangent Line from the menu or toolbar will capture the mouse until you click on a point on a curve to draw the tangent line at.
Keep selecting points with mouse until click on background to stop When checked, the mouse remains captured and you can click on different points to see the tangent line at various positions without re-issuing the Draw Tangent Line command again, until you click on the grid background to indicate that you are done. This option is not meaningful unless you also check "Select initial point/curve with mouse."
Show Draw Tangent Line dialog box This option is checked by default. When checked, the Draw Tangent Line dialog box, which allows you to select an equation and type in the coordinates of the point to draw the tangent line at, is displayed. If you have also checked "Select initial point/curve with mouse," the dialog box will initially be displayed after you select the first point with the mouse. The Draw Tangent Line dialog box allows you to type in the coordinates and select the equation to draw the tangent line for using only the keyboard.