\begin{align} \text{Per a calcular la superfície s hem de} :&\cssId{Step1}{\rho _{600} =\rho _{20}*(1+\alpha *\Delta T)}\\[3px] &\cssId{Step2}{\rho _{600} =10,8·10^{-7}*(1+0,4·10^-3 *(600-20))}\\[3px] &\cssId{Step3}{\rho _{600} =13,3·10^{-7}\Omega ·m}\\[3px] &\cssId{Step4}{R_{600} = n^o_{resistencies}*\rho _{600}*\frac{L}{\pi*\frac{d^2}{4}}}\\[3px] &\cssId{Step5}{R_{600} = 2*13,3·10^{-7}*\frac{3,5}{\pi*\frac{(0,4·10^-3)^2}{4}}}\\[3px] \end{align}