Law of Sines

Sides in proportion to sines

An interactive lesson with 7 steps, about 6 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Law of sines: $\frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C}$ (= the circumcircle's diameter).
• Use it with a side and its opposite angle, plus one more piece.
• SSA data can give two triangles: check the obtuse angle too.

Steps

1. The biggest angle
2. Sides and sines
3. The law of sines
4. Find a side
5. Across the river
6. Two triangles
7. Which angle?