Right triangle: A = a*b/2
Equilateral triangle: A =
Isosceles triangle: $${\mathtt{A}} = {\frac{{\mathtt{1}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}{\mathtt{ah}}$$
For all the other triangles, you basically have to find a height and multiply it by one of the sides of a triangle.
In this case, the height(h) is multiplied by the side (b) and their product divided by 2.
A = b*h/2