# Altitude (geometry)

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In triangle geometry, an **altitude** is a line from a vertex perpendicular to the opposite side. It is an example of a Cevian line. The three altitudes are concurrent, meeting in the **orthocentre**. The feet of the three altitudes form the **orthic triangle** (which is thus a pedal triangle), and lie on the nine-point circle. The area of the triangle is equal to half the product of an altitude and the side it meets.

## References

- H.S.M. Coxeter; S.L. Greitzer (1967).
*Geometry revisited*. MAA. ISBN 0-88385-619-0.