求证:1/(n+1)*(1+1/3+1/5+...+1/2n-1)>1/n*(1/2+1/4+1/6+...+1/2n)
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求证:1/(n+1)*(1+1/3+1/5+...+1/2n-1)>1/n*(1/2+1/4+1/6+...+1/2n)(n>=2且为正数)
二楼确实不对.此题可以运用数学归纳,也可以采用逆推法.要证1/(n+1)*(1+1/3+1/5+...+1/2n-1)>1/n*(1/2+1/4+1/6+...+1/2n),等价于n*(1+1/3+1/5+...+1/2n-1)>(n+1)(1/2+1/4+1/6+...+1/2n),左右各加上n*(1/2+1/4+```````+1/2n),那么左边就是n(1+1/2+1/3+1/4+``````+1/2n),右边是(2n+1)(1/2+1/4+1/6+...+1/2n)=(n+1/2)(1+1/2+1/3+````+1/n),即等价于证明n*(1+1/2+1/3+1/4+``````+1/2n)>(2n+1)(1/2+1/4+1/6+...+1/2n)=(n+1/2)(1+1/2+1/3+````+1/n),消去相同的项,等价于证明[n/(n+1)+n/(n+2)+````+n/(2n)]>1/2*(1+1/2+1/3+````+1/n),只须证明每一项都是前者大于后者,即n/(n+k)>1/2k,这个可以通过真分数不等式,或者直接通分得证.故本题得证
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