Law of Sines

The law of sines establishes the relationship between the sides and angles of an oblique triangle(non-right triangle). Law of sines and law of cosines in trigonometry are important rules used for “solving a triangle”. According to the sine rule, the ratios of the side lengths of a triangle to the sine of their respective opposite angles are equal. Let us understand the sine law formula and its proof using solved examples in the following sections.

What is Law of Sines?

The law of sines relates the ratios of side lengths of triangles to their respective opposite angles. This ratio remains equal for all three sides and opposite angles. We can therefore apply the sine rule to find the missing angle or side of any triangle using the requisite known data.

Law of Sines: Definition

The ratio of the side and the corresponding angle of a triangle is equal to the diameter of the circumcircle of the triangle. The sine law is can therefore be given as,

a/sinA = b/sinB = c/sinC = 2R

• Here a, b, c are the lengths of the sides of the triangle.
• A, B, and C are the angle of the triangle.
• R is the radius of the circumcircle of the triangle.