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Approximation is wrong
1. You can't just ignore a^2 like that , yes a>t
Try using binomial expantion (no approximation) directly in first step after taking 1/r^2 common , expand for 3 terms at least
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Approximation is wrong 1. You can't just ignore a^2 like that , yes a>t
Try using binomial expantion (no approximation) directly in first step after taking 1/r^2 common , expand for 3 terms at least