Schematics: Effect factor and impedance
Could you please explain what parts I've misunderstood and give me any
  ideas to solve it.


The parallel impedance of (L+R) and (C+R) load cannot be simply expressed as: -

\$\dfrac{Z_1 Z_2}{Z_1+Z_2}\$

This is because the phase angles of the currents in each limb are different and without using complex analysis you will be wasting your time.

Imagine what sort of rubbish answer you'd get if you did this for an inductor in parallel with a capacitor at resonance. The right answer is infinite impedance and how you'd get that without respecting \$j\omega\$ (or s) is impossible.