Whenever you have two or more consecutive lines of displaymath, you should seriously consider using the equation-grouping environments of the
amsmath package. The basic equation-grouping environments of this package are
gather, best if each line is a separate entity not particularly related to those that come before or after, and
align, designed to let you align consecutive equations on some location, frequently (but not necessarily) an equal-sign. A line break between equations is created by the
\\ command. There are both numbered and unnumbered versions of these environments -- the latter are created by appending an asterisk to the environment name, e.g.,
align*. The following modified form of your MWE has consistent (and tight) spacing between the equations.
The amsmath package provides additional equation-grouping environments besides the two I've just mentioned, as well as many other commands that are very useful if you need to typeset math-related stuff. Its main user guide is the file amsldoc.pdf.