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n充分大时，1/16＜1/xn＜1/3
|x(n+3)-x(n+2)|=|(1/(x(n+2)^2)-1/(xn^2))|
=|(1/(x(n+2))+1/(xn))((x(n+2)-xn)/
(x(n+2)xn))|＜|2(x(n+2)-xn)/27|
≤|2(x(n+2)-x(n+1))/27|+|2(x(n+1)-xn)/27|，随便选两个k一个奇数一个偶数满足
|x(k+2)-x(k+1)|＜|x(k+1)-xk|
|x(k+3)-x(k+2)|＜4|(x(k+1)-xk|/27
鉴定为柯西序列
|x(n+3)-x(n+2)|=|(1/(x(n+2)^2)-1/(xn^2))|
=|(1/(x(n+2))+1/(xn))((x(n+2)-xn)/
(x(n+2)xn))|＜|2(x(n+2)-xn)/27|
≤|2(x(n+2)-x(n+1))/27|+|2(x(n+1)-xn)/27|，随便选两个k一个奇数一个偶数满足
|x(k+2)-x(k+1)|＜|x(k+1)-xk|
|x(k+3)-x(k+2)|＜4|(x(k+1)-xk|/27
鉴定为柯西序列
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