As you adjust the circle and the secant lines, you will see the angle measures and the arc measures change.[br][br]When the lines intersect INSIDE the circle, the measure of the angles is half the SUM of the intercepted arcs (Theorem 12-13)![br][br]When the lines intersect OUTSIDE the circle, the measure of the angles is equal to half the DIFFERENCE of the intercepted arcs (Theorem 12-14)!

As you adjust the circle and the secant lines, you will see the angle measures and the arc measures change.