In trigonometry, the **law of sines** (or sines law, sine formula) is a statement about arbitrary triangles in the plane.

If the sides of the triangle are A, B and C and the angles opposite to those sides are a, bĀ and c, respectively, then the law of sines states that: .

## Proof

Let a, b, and c be the sides of a triangle opposite the angles A, BĀ and CĀ to side b, perpendicular to each other. This divides the original triangle into two right triangles. We let X be the length of this dividing line.

Since sine is opposite divided by hypotenuse, then

and and

Solving for X in the first two equations,

and

By cross products,

But solving for *a*,

and

Combining the resulting equations, we get