\input amstex

$$\aligned
\hbox{arg}\,\pmb{\hbox{i}}&=\pi/2\\
\hbox{arg}(\pmb{-\hbox{i}})&=\frac32\pi=-\pi/2\\
\hbox{arg}(1+\pmb{\hbox{i}})&=\pi/4=-\hbox{arg}(1-\pmb{\hbox{i}})\\
\endaligned$$
\bye