Define the definite integral M(a, b) = J f(x) dx to equal the .shaded a curvilinear trapezoid bounded by y = fCx); the straight lines x = a, x=b and the horizontal axis. The definite integral M(a, b) depends, (1) on the form of the function f(x), (2) the lower limit of integration a, (3) the upper limit of integration b. The definite integral MCa^i b) does not depend on the variable of integration. The latter can be denoted by any letter; i.e. we can replace x by any other letter without affecting the value MCa, b)
Wetering, H. Ike Van de, "The dual industry benefit loss function and extensions of the Harberger policy evaluation rule" (1989). Economic Staff Paper Series. 202.