Exterior Angle of a Triangle

When one side of a triangle is extended, the angle formed outside it is an exterior angle. In NSW Year 7 maths (NESA outcome MA4‑GEO‑C‑01) you learn that it equals the total of the two interior opposite angles, the angles not next to it.

You will use this rule to find exterior and interior angles, work backwards from a known exterior angle, combine it with angles on a straight line and the angle sum, handle isosceles triangles and pronumerals, and follow a short proof of why the rule holds. It is a handy shortcut when checking angles in roof frames and bridge trusses. It is part of Year 7 Geometry.