So, my little "proof", is correct, and "rigorously" we should have \int[f(x)+g(x)]dx = \int f(x)dx + \int g(x)dx + C[SIZE="3"]?
Altough, the Constant C doesn't do any difference as a consequece to definite integrals.
So, I can think that Spivak didn't write the C, because of this...