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 D are equal to cosine and sine of α + β, α, and –β, respectively. Write expressions for the lengths and set them equal. Rewrite cos(-β) and sin(-β) using the identities before squaring both sides of the equation. Simplifying the resulting expressions involving cosine and sine of α, β, and α + β requires using the identity. When sim