Angle Sum of a Triangle

The three angles inside any triangle always add to \(180^\circ\). This NSW Year 7 maths subtopic (NESA outcome MA4‑GEO‑C‑01) uses this angle sum to find missing angles and shows why it is true with a line drawn parallel to one side.

You will check whether three angles could belong to a triangle, find a third angle, read angle names such as \(\angle ABC\), use equal base angles in isosceles triangles, start with angles on a straight line when needed, solve equations such as \(x + 2x + (x + 20) = 180\), and give a reason for each step. A ladder leaning on a wall is one real example. It is part of Year 7 Geometry.