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
1x1/3+1/3x1/5.+1/19x1/21+1/2x1/4
=1/2×(1-1/3)+1/2×(1/3-1/5)+……+1/2×(1/19-1/21)+1/2x1/4
=1/2×(1-1/3+1/3-1/5+……+1/19-1/21)+1/8
=1/2×(1-1/21)+1/8
=10/21+1/8
通分就行了,不知道你这个+1/2x1/4是不是多写了
1x1/3+1/3x1/5.+1/19x1/21+1/2x1/4
=1/2×(1-1/3)+1/2×(1/3-1/5)+……+1/2×(1/19-1/21)+1/2x1/4
=1/2×(1-1/3+1/3-1/5+……+1/19-1/21)+1/8
=1/2×(1-1/21)+1/8
=10/21+1/8
通分就行了,不知道你这个+1/2x1/4是不是多写了
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