z=1-x/i - 3i
=1-xi/i²-3i
=1+xi-3i
=1+(x-3)i
因为是纯虚数,所以：x-3=0
x=3
如果：
1-i分之x-3i, 1-i是整体做分子,x-3i是整体做分母,那么：
z=(x-3i)/(1-i)
=(x-3i)(1+i)/(1-i²)
=(x+xi-3i-3i²)/(1+1)
=[x+3+(x-3)i]/2
因为是纯虚数,所以：x-3=0
x=3