Trigonometry

Right-angled triangles only get you so far. This standard (AS91259) is the toolkit for any triangle and for circles: the sine rule and cosine rule for sides and angles, the area of a triangle from two sides and the angle between them, and arc length and sector area for slices of a circle. The real skill is choosing the right tool for the information you have — and chaining two or three of them together in a real context such as surveying, navigation or design.