你好!
(3-i)/(1+2i)
=(3-i)(1-2i)/(1+2i)(1-2i)
=(3-6i-i+2i^2)/(1-4i^2)
=(1-7i)/5
=1/5-7i/5
所以b=-7/5
a=1/5
所以b/a=-7