Usually called "half of base times height", the area of a triangle is given by the formula below.Area=ba2whereb is the length of the base
a is the length of the corresponding altitude
You can choose any side to be the base. It need not be the one drawn at the bottom of the triangle. The altitude must be the one corresponding to the base you choose. The altitude is the line perpendicular to the selected base from the opposite vertex.
In the figure above, one side has been chosen as the base and its corresponding altitude is shown.