Calculating Change in Electric Potential Energy with Multiple Charges

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The discussion centers on calculating the change in electric potential energy when moving a -2.5uC charge between two points influenced by two 33uC charges. The user applied the formula U=kq1q2/r to find the initial and final potential energy values. They calculated a change of Uf - Ui = 1.74J but received feedback indicating this result is incorrect. The user seeks clarification on their calculations and any potential mistakes made in the process. Accurate calculations are essential for determining the correct change in electric potential energy.
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Homework Statement


A 33uC point charge is placed 36cm from an identical 33uC charge. A -2.5uC charge is moved from point a to point b in the figure:
http://session.masteringphysics.com/problemAsset/1096949/4/GIANCOLI.ch17.p72.jpg


Homework Equations


U=kq1q2/r


The Attempt at a Solution



I found Ui by using above equation for each r from the -2.5uC charge and found the difference.

Repeated for Uf when charge is moved from a->b.

Uf-Ui = 1.74J, but MP says this is incorrect. What am I missing?
 
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