( E ) ∑ 1 ⩽ i , j ⩽ n a i j ∂ 2 f ∂ x i ∂ x j + ∑ i = 1 n b i ∂ f ∂ x i + c f = g {\displaystyle (\mathbf {E} )\qquad \qquad \sum _{1\leqslant i,j\leqslant
\over R_{n}}} 假設在電路兩端的電壓為 V {\displaystyle V} ,則通過的电流為 I = V / R t o t a l {\displaystyle I={V}/{R_{total}}} 。假設每一個電阻器都遵守歐姆定律,則這電路是電阻為 R t o t a l {\displaystyle