Trigonometric functions

1  Trigonometric function:
 2      Known: a triangle has three angles, ∠A, ∠C, ∠B, where ∠C is a right angle
 3      The opposite sides of ∠A, ∠C, ∠B are a, c, b
 4      sin = a / c
 5      cos = b / c
 6      tan = a / b
 7      cot = b / a
 8      sec = c / b
 9      csc = c / a
 10      
11  The relation between circle and trigonometric function:
 12      In Cartesian coordinate system: x0 ,y0 = 0,0
 13      x0,y0 = coordinates of the center of the circle
 14      r = radius of the circle = 12
 15      The coordinates in the 30-degree direction:
 16          x1 = r * sin(30 * pi / 180 )
 17         y1 = r * cos(30 * pi / 180 )
 18      
19      Analysis:
 20          pi = pi
 21          circle length = r * 2 * pi
 22          then degree scale = degrees * 2 * pi / 360 = degrees * pi / 180