Calculate the area of a triangle?

If it is a right angled triangle then it is 0.5*(base)*(height) of the triangle half the base times by the height imagine it as half the area of a rectangle of the two triangles

* * * * *

That is all very well if you know the base and the height. Sometimes you know only the three sides. Or only two sides and an angle, or two angle and a side. There are a whole host of formulae for such circumstances.