Integral of f1(x) from xa to xb is area below curve f1(x) between xa and xb
Integral of f2(x) from xa to xb is area below curve f2(x) between xa and xb
Integral of f2(x) - f1(x) from xa to xb is area between curve f2(x) and f1(x) between xa and xb
Annulus sector can be calculated from integral of f2(x) - f1(x) from xa to xb, where xa = ra and xb = rb.