\begin{align} \text{Per a calcular la R hem de:} :&\cssId{Step1}{R =\rho*\frac{L}{s}}\\[3px] &\cssId{Step2}{s = \pi*r^2}\\[3px] &\cssId{Step3}{r = \frac{d}{2} }\\[3px] &\cssId{Step4}{s = \pi*\frac{d^2}{4} }\\[3px] &\cssId{Step5}{R_{20} = n^o_{resistencies}*\rho*\frac{L}{\pi*\frac{d^2}{4}}}\\[3px] &\cssId{Step6}{R_{20} = 2*10,8·10^-7*\frac{3,5}{\pi*\frac{(0,4·10^-3)^2}{4}}}\\[3px] \end{align}