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The problem asks, "What is the electrical potential energy U of the pair of charges?"

Okay, so I tried using the formulas:

**-(Ub-Ua)=-deltaU**and

**U=((1)/(4*pi*Eo))*((q1*qo)/(r))**

Where I'm confused is that q1 and qo appear the same in both Ua and Ub, thus wouldn't Ub-Ua cancel and equal zero? Here's my obviously incorrect work:

-(((1)/(4*pi*Epsilono))*(((1.2x10^(-9))(4.6x10^(-9)))/(.25m)) = -1.98x10^(-7)

However, the correct answer according to the back of the book is "0.198J." Can anyone with patience please explain what I'm doing wrong? This material is new to me. I'm in a 5 week summer calculus 2 based engineering physics 2 course (PHY 122). We only just started talking about this material yesterday.