Circle

 If line segment AC is tangent to circle B at a, the line segment AC is perpendicular  to line segment BA.

-Draw a line segment from center of the circle to a point on the line.
-Measure the distance of the radius.
-Measure from the point on the line to the point the on the circle.
-Use Pythagorean Theorem to find the missing side length in the triangle.
-If the line is Tangent then by using the Pythagorean Theorem the triangle should be a right triangle.
-The right angle should be at the point were the Tangent line hit the circle, in this example it is shown as A.