why we use there the formula of triangle in

Displacement = Area under the velocity - time graph. =12×Base×height =12×8×4 = 16 m

It always needn't be the formula of a triangle,the formula depends on the shape which will be made by the graph.if it is a undefined shape,then you have to divide it into recognizable shape by drawing an imaginary line and choose the formula of that shapes and add them.