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## Homework Statement

The potential difference across the terminals of a battery is 8.20 V when there is a current of 1.54 A in the battery from the negative to the positive terminal. When the current is 3.56 A in the reverse direction, the potential difference becomes 9.40 V. What is the internal resistance r of the battery?

## Homework Equations

V_ab = E - Ir

## The Attempt at a Solution

So I thought that it would make no difference if the charge was reversed. So I just plugged the values into the equations, so since I have two equations and two variables I could solve for r.

8.56 = E - 1.54r -> E = 8.56 + 1.54r

9.40 = E - 3.56r -> E = 9.40 + 3.56r -

0 = -.84 - 2.02r

r = -.416

Obviously the resistance can't be negative. What am I doing wrong, does it have something to do with the reversing part of the problem?