作业帮1x1/3+1/2x1/4+1/3x1/5.+1/19x1/21用简便方法计算等于多少
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1x1/3+1/2x1/4+1/3x1/5.+1/19x1/21用简便方法计算等于多少
1x1/3+1/2x1/4+1/3x1/5.+1/19x1/21用简便方法计算等于多少
1x1/3+1/2x1/4+1/3x1/5.+1/19x1/21用简便方法计算等于多少
1x1/3+1/2x1/4+1/3x1/5.+1/19x1/21
=1/2x(1-1/3+1/2-1/4+1/3-1/5+……+1/19-1/21)
=1/2x(1+1/2-1/20-1/21)
=1/2x589/420
=589/840