what is the the power dissipated by an ideal inductor in AC circuit? explain?

prove that the power dissipated in on ideal resistor connected to an AC source is (V(eff))^2 / R

Three equal resistors connected in series across a source of emf E dissipate total power of 90 watts. Find the total power dissipated if the same resistors are connected in parallel across the same source of emf.