Given a circle and a point **P**(x, y) of the plan, we have:

a) if **P** belongs to the circumference, so there is a single line tangent to the circumference by **P**.

b) if **P** is outside the circumference, so there are two lines tangent to it by **P**.

c) if **P** is inside the circumference, so there is no tangent line to the circumference passing through the point **P**.