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If you just want an approximate answer, you should just send r to zero. The answer you gave contains a term proportional to ##r^4##. This term is negligible compared to the term you are missing due to not including the offset.Bling Fizikst said:It should be 'approximately' true i guess as ##r## is given to be small . Hence , the second order term ##r^2\approx 0## . So , i guess we can only get an approximate answer .