Question 208787
(e/R) +r =I,,,,,subst  r
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e/R = I-r,,,,,,,,divide  thru  by (I-r),,,,multiply  thru  by  R
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e/(I-r) = R
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If  problem  is  e/(R+r) = I,,,,,,,,ccross  multiply
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e = (R+r)I,,,,,,divide  thru  by  I
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e/I = R+r,,,,,,,substract  r
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(e/I) -r =R
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