sine of angle of triangle
The cosines law allows to express the cosine of an angle of triangle through the sides:
(1) |
Substituting this to the “fundamental formula of trigonometry”,
we can calculate as follows:
Thus we have the beautiful formula
Substituting (1) similarly to the general formula for the sine of half-angle (http://planetmath.org/GoniometricFormulae)
one can obtain the formula