Respuesta :

so the conjugate will be 1-2i 

(1+2i)(1-2i) = 1 -(2i)^2 = 1 - 4(-1) = 1+4 = 5 

hope this will help you 
[tex]\overline{1+2i}=1-2i\\\\ (1+2i)(1-2i)=1+4=5[/tex]