Sine is the ratio of strands to chords, and cosine is the ratio of remaining right angles to chords.

Pythagoras' chords are placed in a circle, and the chords are connected by two points on the circle. The largest chord is the diameter. If the chord of a right triangle is placed on the diameter, the chord is the inverse chord of ∠A, that is, sine, and the hook is the remaining chord-cosine.

Properties of sine function:

(1) Maximum and Zero

① Maximum value: when x=2kπ+(π/2) and k∈Z, y(max)= 1.

② Minimum value: when x=2kπ+(3π/2) and k∈Z, y(min)=- 1.

Zero: (kπ, 0), k∈Z

(2) Symmetry

It is not only an axisymmetric figure, but also a centrally symmetric figure.

1) Symmetry axis: symmetrical about the straight line x=(π/2)+kπ, k ∈ z.

2) Central symmetry: about point (kπ, 0), k∈Z symmetry.