Pure · Trigonometry
Chapter 1 · 3
The idea
The sine and cosine rules
Solving any (non-right-angled) triangle — the sine rule, the cosine rule and the area formula ½ab·sinC — and how to choose the right one from what you're given.
A full journey — read it, play with it, work it, then earn real exam marks. Everything stays on the timeline below.
In this lesson — start anywhere
Pure · Trigonometry
The sine and cosine rules
Solving any (non-right-angled) triangle — the sine rule, the cosine rule and the area formula ½ab·sinC — and how to choose the right one from what you're given.
Why it works
A triangle with no right angle
Here is the whole lesson in one question: a triangle has sides and enclosing an angle of — solve it. Find the third side, the other two angles, the area. SOH-CAH-TOA is no help, because it only works in right-angled triangles, and this one has none. Two rules and one formula take over, and this triangle will tour all three. First, the labelling everything depends on: each side takes the lower-case letter of the angle opposite it — side faces angle , side faces , and so on.Keep reading — free
The rest of the explanation, plus 3 worked examples you step through move by move.
Start freeTakes a minute — no card.