The formula of sine theorem: a: b: c = Sina: sinb: sinc. Sine theorem is a basic theorem in trigonometry, which points out that "in any plane triangle, the ratio of sine value of each side to its diagonal is equal, and it is equal to the diameter of the circumscribed circle". The sine theorem points out the relationship between the three sides of an arbitrary triangle and the sine value of the corresponding angle. According to the monotonicity of sine function in interval, sine theorem describes a quantitative relationship between the sides and angles of any triangle.