Brhma Gupta formula is for calculating the area of a cyclic quadrilateral. If all the four vertices of quadrilateral are points on a circle, then it is a cyclic quadrilateral.
Rectangle, square are also cyclic quadrilaterals.
If a, b, c and d are sides of a cyclic quadrilateral then,
semi-perimeter is s = (a + b+ c + d ) / 2
This is the generalization of the area of triangle that we are aware of.