An Altitude is a line segment:
Whose one end is at vertex of a triangle
Whose other end is on the opposite side of aforementioned vertex
Which is perpendicular to this opposite side
Example: In the following Triangle ABC:
AD is a line segment:
Whose one end is at vertex 'A' of a triangle
Whose other end 'D' is on the opposite side 'BC' of vertex 'A'
Which is perpendicular to the opposite side 'BC'
Hence, AD is an altitude of Triangle ABC.

Study More Solved Questions / Examples
