Improper Integrals The Fundamental Theorem of Calc. (FTC) gives that if f: [a;b] !R iscontinuous on [a;b], then Z b a f (x) dx = F (b) F (a) where F is any antiderivative of f, i.e., a function such that F0 = f. Note that to apply the FTC, one needs f to be continuous on the whole interval [a;b], i.e., we need the following four conditions: