How equation#4 derived from the V/T graph

In Equation #4, only V2 is usable. In another way to calculate the area of the trapezoid, we could calculate the area of the trangle and subtract the area of the triangle.   That would come out the equation #4, which is d=V2Δt-½aΔt².

Equation #4  ────  d=V2Δt - ½aΔt²
                                   ↓            ↓
                              Rectangle    Triangle


To determine the area of the rectangle, we would multiple V2 (as symbol "b" shown in the graph《Height》)  to t2 - t1, which known as Δt. (as symbol "h" shown in the graph《Height》)

It will give us an equation,
Area of Rectangle = V2Δt

To determine the area of the rectangle, we would multiple (V2 - V1) 《Base》to  Δt 《Height》divided by 2.

According to Equation #1  = V2 - V1, so, as we sub this equation,
                                         V1= V2 -  aΔt

Area of triangle = ½(V2 - V1)(Δt)
Area of triangle = ½(V2 - V2 - aΔt)Δt
Area of triangle = ½(- aΔt)Δt
Area of triangle = -½aΔt²

Displacement = Area of rectangle - Area of triangle (As we sub the equations in,)

Displacement = V2t + ( -½aΔt²)

Displacement = V2t - ½aΔt²