正弦定理是三角学中的一个定理。它指出：对于任意，、、分别为、、的对边，为的外置圆半径，则有

做一个边长为a，b，c的三角形，对应角分别是A，B，C。从角C向c边做垂线，得到一个长度为h的垂线和两个直角三角形。

很明显：

和

因此：

和

同理：

In trigonometry, the law of sines, sine law, sine formula, or sine rule is an equation relating the lengths of the sides of any shaped triangle to the sines of its angles. According to the law,

where a, b, and c are the lengths of the sides of a triangle, and A, B, and C are the opposite angles (see the figure to the right), while d is the diameter of the triangle's circumcircle. When the last of these equations is not used, the law is sometimes stated using the reciprocals;