The Trapezoid Integration Method uses right trapezoids to approximate the curve. The area of these trapezoids can be broken down into the area of a square plus the area of a triangle:
f(x)*dx + 0.5*(f(x)-f(x+dx))*dx, which simplifies to:
0.5*(f(x)+f(x+dx))*dx
Notice that the Trapezoid Integration Method is numerically equivalent to the Average Riemann Sum.