A triangle on a sphere is composed of points $A$, $B$ and $C$. The $\alpha$, $\beta$ and $\gamma$ denote the angles at the corresponding points of the triangle:
The Girard's theorem states that the surface area of any spherical triangle:
$$ A = R^2 \cdot E $$
where $R$ is the radius of the sphere and $E$ is the excess angle of $(\alpha + \beta + \gamma - \pi)$
I'm wondering how to derive this formula.
Could you please help explain this clearly?