There are some ways to calculate the area of the trapezium. We can easily calculate the area of the trapezium by using the length of the parallel sides and the vertical distance between them. We can also use Brahmagupta's formula and Heron's formula. But in this article I will use the length of one non parallel side and the vertical distance between this non parallel side (or the increased part of this non parallel side) and the midpoint of the other non-parallel side. For this at first I have to prove a theorem and the theorem is, if we imagine one non parallel side of the trapezium as the base of a triangle and the midpoint of other non-parallel side as the vertex of that triangle, the area of the triangle will be half of the area of the trapezium. After proving this theorem, we will easily get a new formula to calculate the area of the trapezium.