From Förberedande kurs i matematik 1

Because the triangle is isosceles (two sides have the same length), it can be divided up into two right-angled triangles of the same size by introducing a side which divides the angle \displaystyle v in half.

If we look at one of the triangles, we can set up the trigonometrical relation

\displaystyle \text{sin }\frac{v}{2}=\frac{1}{3},

which is an equation for \displaystyle v.