Tangents to a Circle

Thales to the rescue

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• A tangent is perpendicular to the radius at the touching point.
• From an external point P, use the circle on diameter OP (Thales) to find the touching points.
• There are two tangents from an external point, of equal length.

Steps

1. Touching, not crossing
2. Find the touching point
3. Where it touches
4. Tangent length
5. Angle between tangents
6. Two tangents