The newtxmath documentation says that it loads amsmath now. However, mathtools also loads amsmath. What should the order of the packages better be then:
\usepackage{newtxtext}\usepackage{mathtools}\usepackage{newtxmath}or
\usepackage{newtxtext}\usepackage{newtxmath}\usepackage{mathtools}?
(So far, I've been loading mathtools last, but my texts are big, so small changes/errors during updates/upgrades sometimes go unnoticed.)We may safely assume that the user passes some options to all of the three packages.