Definition: sin: In a right triangle, the ratio of the opposite side to the hypotenuse of ∠α (non-right angle) is called the sinα of ∠α, that is, the hook-chord ratio. The "string" in the ancient "hook three strands, four strings and five" is the hypotenuse in the right triangle. The thigh is a human thigh. The ancients called the long right side of a right triangle "thigh".

Relationship between angle functions

(sine) Sin θ = opposite side A/ oblique side c

(cosine) Cosθ = adjacent side B/ oblique side c

(tangent) Tanθ = opposite side A/ adjacent side b

Opposite side A = hypotenuse C * Sinθ

Opposite side A = adjacent side B * Tanθ

Adjacent side B = hypotenuse C * Cosθ

Adjacent side B = opposite side A/Tanθ.

Hypotenuse C = opposite side A/Sinθ

Oblique side C = adjacent side B/Cosθ