左=1-1/2+1/3-1/4+……+1/(2n-1)-1/2n
=1+1/2+1/3+1/4+……+1/(2n-1)+1/2n - 2(1/2+1/4+1/6+1/8+...+1/2n)
=1+1/2+1/3+1/4+……+(1/2n-1)+1/2n- [1+1/2+1/3+1/4+...+1/(n-1)+1/n]
=1/(n+1)+1/(n+2)+1/(n+3)+...+1/(2n-1)+1/2n
=右
所以,得证
求证:1-1/2+1/3-1/4+...-1/2n=1/(n+1)+1/(n+2)+...+1/2n
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