TITLE: What is the least number of (fixed) parameters I can ask for, when calculating area of a triangle of unknown type?
QUESTION [5 upvotes]: I need to calculate the area of a triangle, but I don't know, whether it is right angled, isoscele or equilateral.
What parameters do I need to calculate the area of a triangle of unknown type?

REPLY [6 votes]: The area of a triangle can also be calculated as $\frac12 rp$, where $r$ is the radius of the incircle and $p$ is the perimeter.