11x2+12x3+13x4+14x5.+.+.+.+.+11999x2000
=（1-1/2）+（1/2-1/3）+（1/3-1/4）++.+（1/1999-1/2000）
=1-1/2000
=1999/2000
1\1x2+1\2x3+1\3x4+1\4x5.+.+.+.+.+1\1999x2000
4个回答
相关问题
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