Proofs
We said earlier that it is more important to understand a mathematical statement than to understand its proof. We have put some of the harder proofs in Appendix A; these can safely be skipped by a student studying multivariate
calculus for the first time. We urge you, however, to read the proofs in the
main text. By reading many proofs you will learn what a proof is, so that (for
one thing) you will know when you have proved something and when you have
not.