In ancient times, the chords in the hook, three strands, four strands and five strands were the hypotenuse of the right triangle, and the hook and strand were the two right-angled sides of the right triangle. Sine, a mathematical term. In a right triangle, the ratio of the opposite side to the hypotenuse of any acute angle ∠A is called the sine of ∠A, and it is recorded as sinA, that is, the opposite side/hypotenuse of Sina = ∠ A. In ancient times, sine was the ratio of chord to chord.