# Relation between area of a triangle on a sphere and plane

We know area of a plane triangle $\Delta=\sqrt{s(s-a)(s-b)(s-c)}$ where $s=\frac{a+b+c}{2}$.

I was just thinking: let we have a triangle with arc length $a,b,c$ on a sphere of radius $r$, do we have any similar kind of formula for that spherical triangle? when radius of $r\to \infty$ we get the plane, so do we have any estimate of area of spherical triangle when $r\to\infty$?

Any reference and article link are also welcome! Thank you.

$$\frac{ 180 \cdot \sqrt{ s(s-a)(s-b)(s-c) } } { 4 \cdot \pi \cdot R^2 \cdot \arctan \left( \sqrt{ \tan \left( \frac{s}{2} \right) \cdot \tan \left( \frac{s-a}{2} \right) \cdot \tan \left( \frac{s-b}{2} \right) \cdot \tan \left( \frac{s-c}{2} \right) } \right) }$$
Basically, I just placed Heron's formula for the area of planar $\Delta$s above the $\frac{ \pi \cdot R^2 \cdot E}{180}$ formula for the area of a spherical $\Delta$. Since I didn't have the angular measures required to calculate $E$, I used this formula:
$$\tan \left( \frac{E}{4} \right) = \sqrt{ \tan \left( \frac{s}{2} \right) \cdot \tan \left( \frac{s-a}{2} \right) \cdot \tan \left( \frac{s-b}{2} \right) \cdot \tan \left( \frac{s-c}{2} \right) }$$