The area of the right angled triangle as well as that of the isosceles triangle is calculated as
Area = 1/2 (b * h)
The explanation is logical, observe the right angled triangle (the one on the left) and you'll see that the length covered by the height (labelled as h) is not the entire width covered by the base (labelled as b) unlike what you have in a rectangle or square. Its only logical to multiply the base by half of the height, otherwise you might end up calculating the area of a rectangle.
That applies to all triangles in general, the area is calculated as