(2+7i)(2-7i)= ==========
\((2+7i)(2-7i)=4-14i+14i-49i^2\\ \\ 4+49=53\)
\((2+7i)(2-7i)=2^2-(7i)^2=4-49\vfot(-1)=4+49=53\\\\-------------------------------\\i=\sqrt{-1}\\\\i^2=-1\)
