解“：z=1-2i/3+4i
=(1-2i)(3-4i)/(3+4i)(3-4i)
=(3-4i-6i+8i)/(9-16i)
=(3-10i-8)/(9+16)
=(-5-10i)/25
=-1/5--2/5i
∴a=-1/5,b=-2/5.
楼主请采纳!谢谢!