\begin{align} &\cssId{Step1}{R = {{{U_{1}}^2}\over{P_{1}}} = {{{U_{2}}^2}\over{P_{2}}}}\\[3px] &\cssId{Step2}{P_{2} = {{{{U_{2}}^2} · {P_{1}}}\over{U_{1}}^2}}\\[3px] &\cssId{Step3}{P_{2} = {{{{125}^2} · {900}}\over{230}^2} = 265,83W}\\[3px] \end{align}

1. Igualem R per a \(U_{1}\) i \(U_{2}\).
2. Aïllem \(P_{2}\).
3. Sustituïm les dades pels seus valors.