Solució: \begin{align} P_{llum} &= \cssId{Step1}{ \frac {U^2}{R}} \\[3px] &\cssId{Step2}{ R = \frac {U^2}{P_{llum}}} \\[3px] &\cssId{Step3}{ R_{eq} = \frac {R}{2}} \\[3px] \end{align}