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Homework Statement
http://www.aijaa.com/img/b/00334/3685081.jpg
Homework Equations
[tex]B= \frac{ \mu _0}{2 \pi r}I[/tex]
The Attempt at a Solution
I got it wrong the two first times (I got [tex]\frac{ \mu _0I}{2 \pi a}(1- \frac{a}{b})[/tex]), then the assistant teacher showed how it was done, but wasn't able to tell me why. This is the answer I was showed: [tex]B=B_a-B_b= \frac{ \mu _0I}{4}( \frac{1}{a}- \frac{1}{b})= \frac{ \mu _0I}{4a}(1- \frac{a}{b})[/tex] Can anybody clarify what the heck happened between steps 2 and 3, everything else I can understand.
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