You will need
  • Know the angles of a triangle and at least one of its sides.
Method 1. Out of the theorem of sines of a triangle. The theorem of sines States that: the sides of the triangle are proportional to the sines of the opposite angles (Fig. 1)
From this formula implies the following equality:a = 2Rsinα,b = 2Rsinβ
Fig. 1. R is the radius of the circle circumscribed around the triangle.
Method 2. Out of the cosines of the triangle. According to this theorem, for any plane triangle with sides a, b, c and angle α, which lies opposite the side, true equality in Fig. 2
Hence, there is a consequence:a = b/2cosα;
Also from the theorem of cosines, there is 1 more result:
b = 2a*sin(β/2)
How to find <b>side</b> isosceles triangle