Series: $R_{eq}$ = R1 + R2 + ... + $R_n$. Same current I through all. Voltage divides: $V_i$ = $IR_i$. Total V = sum($V_i$). Parallel: 1/$R_{eq}$ = 1/R1 + 1/R2 + ... + 1/$R_n$. Same voltage V across all. Current divides: $I_i$ = $\frac{V}{R_i}$. For two resistors in parallel: $R_{eq}$ = $\frac{R1R2}{R1+R2}$. Current divider: I1 = $I_{total}$ * $\frac{R2}{R1+R2}$. n identical resistors R: series gives nR, parallel gives R/n. Series increases total resistance; parallel decreases it. Mnemonic: series = same I, parallel = same V. Capacitor combinations are opposite.