∫f(x)dx≈b-a/n *[f(x0)/2+f(x 1)+f(x2)+f(x3)..+f(xn- 1)+f(xn)/2 ]

The upper limit of integral is b, a.

The concrete point is to put the closed interval A and B of X on the X axis as two endpoints, and f(x) is the curve of Y, then decompose the part under the curve into trapezoid, and A to B into n parts. If the parts are equal, the variation of Y with respect to the parts of X is equal, and then the area of each trapezoid is calculated and added.