[tex]\bf \textit{difference of squares} \\\\ (a-b)(a+b) = a^2-b^2 \\\\[-0.35em] ~\dotfill\\\\ (9+2i)(9-2i)\implies [9^2~-~(2i)^2]\implies [81-(2^2i^2)]\implies [81-\stackrel{i^2 = -1}{(4\cdot -1)}] \\\\\\ 81-(-4)\implies 81+4\implies 85[/tex]
