Proof of b):
Cubic expansion gives (4 + 1/n^2)^3 = 64 + 48/(n^2) + 12/(n^4) + 1/(n^6),
which, in view of 1/(n^6) < 1/(n^4) < 1/(n^2), yields
|(4 + 1/n^2)^3 - 64| < 61/(n^2) < 61/(61/epsilon) = epsilon
for n > sqrt{61/epsilon}.
Proof of b):
Cubic expansion gives (4 + 1/n^2)^3 = 64 + 48/(n^2) + 12/(n^4) + 1/(n^6),
which, in view of 1/(n^6) < 1/(n^4) < 1/(n^2), yields
|(4 + 1/n^2)^3 - 64| < 61/(n^2) < 61/(61/epsilon) = epsilon
for n > sqrt{61/epsilon}.
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谢谢大侠出手相救
-woodwishper-
♂
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09/29/2017 postreply
00:41:28