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This was never an issue. The relation ##v=c(1-\epsilon)## from (a) is what defines ##\epsilon## and it is therefore exact. He only needs to keep terms to order ##\epsilon^2## when adding the velocities, which he did.PeroK said:n the first method you used the approximation 1−ϵ1−ϵ1-\epsilon in the calculations for part b) and dropped the term in ϵ2ϵ2\epsilon^2 (which wasn't needed for part a). If you put that term back in, you should get the same answer as in the second method.