The second part of the fundamental theorem of calculus is my favorite. Let f be a continuous real-valued function defined on a closed interval [a,b]. Let F be an antiderivative of f, that is one of the indefinitely many functions such that, for all x in [a,b], $ f(x)=F'(x) $ then $ int(f(x),x,a,b)=F(b)-F(a) $
