Triangles AOC and BOD are congruent by SAS. Therefore, and these lengths can be found using the . Because this is the unit circle, the coordinates of points A, B, and C are equal to cosine and sine of , – β, and β respectively. Write expressions for the lengths, set them equal, and square both sides of the equation. Simplifying the resulting expressions involving cosine and sine of , β, and – β requires using the identity. When simplified, the equation becomes cos( – β) = cos()cos(β) + sin()sin(