Calculate the area, perimeter, and type of a triangle from its three sides.
Heron's formula calculates the area of a triangle when you know all three side lengths, without needing the height.
Equilateral (all sides equal), isosceles (two sides equal), and scalene (all sides different).
A triangle is invalid when any side is greater than or equal to the sum of the other two sides. This is called the triangle inequality.