Sine Theorem

Theorem

The sides of the triangle are proportional to the sines of the opposite angles.



Evidence.

Let there be Δ ABC with sides a, b, c and angles α, β, γ.
Prove that



We will carry out from CD height CD. Then from Δ ACD we get:



If angle α is blunt, then



From Δ BCD we get



Similarly, we get



The theorem is proved.