Trigonometry/Heron's Formula
Heron's formula states that the area A of a triangle whose sides have lengths a, b, and c is
where is the semiperimeter of the triangle:
There is a proof here.
Worked Example todo: add picture
Let's see if Heron's formula works too. And using Heron's formula: It worked. 
The formula is believed to be due to Hero (or Heron) of Alexandria (10 – 70 AD), a Greek mathematician.
Exercise todo: add picture
so the area of our equilateral triangle is .
And using Heron's formula:
