b: The length of a base (side) of a triangle

a: The length of the altitude of the triangle (corresponding to the chosen base)

A: The area of the interior of the triangle (Explanation of formula).

Click here to see an applet that demonstrates the area formula.

• Altitude(s) of a triangle: a segment from a vertex of the triangle, perpendicular to the side opposite that vertex of the triangle.
• Angle bisector(s) of a triangle: a segment, ray or line which divides an angle of the triangle into two congruent (equal in measure) parts.
• Median(s) of a triangle: a segment from a vertex of the triangle, to the midpoint of the side opposite that vertex of the triangle.

Right triangles are labelled according to a chosen reference angle. The hypotenuse (h) is labelled first, then the legs are labelled as opposite (o) or adjacent (a), depending on which remaining angle is chosen as the reference angle.