Apply the power-of-a-point theorem for intersecting chords, a tangent and secant, or two secants from an external point. Give three known lengths and find the fourth.
t = tangent length; p = external secant segment; q = whole secant length (p + chord inside).
p₁, p₂ = external segments; q₁, q₂ = whole secant lengths from external point.
Intersecting chords: AE·EB = CE·ED
Tangent-secant: t² = p·q
Two secants: p₁·q₁ = p₂·q₂
All three cases are versions of the Power of a Point theorem. The product of signed distances from a fixed point to any two intersection points with a circle is constant — regardless of which line you draw through that point.
A tutor can explain power of a point and every chord, secant, and tangent theorem with clear diagrams.