解由z=2m+i/1+2i
=(2m+i)(1-2i)/(1+2i)(1-2i)
=(2m-4mi+i-2i²)/5
=(2m+2-4mi+i)/5
=(2m+2+(1-4m)i)/5
由z是纯虚数
即2m+2=0
即m=-1