Suppose z=1/(a+i*b)
Then z not equal to 1/a + i*1/b
Why? Basic fraction calculus. You cannot "split up" a fraction among its denominator, which is what you're intending. You can split it up among the nominator though, so you should make the denominator real-valued by expanding with its complex conjugate, that is
z=(a-i*b)/((a+i*b)*(a-i*b))=(a-i*b)/(a^2+b^2)
so your final expression would be
a/(a^2+b^2)-i*b/(a^2+b^2)
If you don't understand the above, my advice would be to real a bit about basics of complex numbers on wikipedia (no offense)