For resistors in series, the equivalent resistance is —

• a$R = R_1 + R_2 + R_3 + \ldots$
• b$\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \ldots$
• c$R = R_1 R_2 R_3$
• d$R = R_1 - R_2$