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Explore the relationship between trig functions of complementary angles. sin(θ) = cos(90° - θ).
Why "cofunction"? The "co" in cosine, cotangent, and cosecant stands for "complement." Each function equals its cofunction of the complementary angle.
In a right triangle, the two acute angles are complementary (add to 90°). What is "opposite" for one angle is "adjacent" for the other. This is why sin(θ) = opposite/hypotenuse equals cos(90°-θ) = adjacent/hypotenuse.