已知:2/1×3=1/1-1/3,求①2/1×3+2/3×5+2/5×7+2/7×9+2/9×11的值;
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2/1×3+2/3×5+2/5×7+2/7×9+2/9×11

=1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11

=1-1/11

=10/11

1/1×3+1/3×5+1/5×7+1/7×9+1/9×11

=(2/1×3+2/3×5+2/5×7+2/7×9+2/9×11)÷2

=(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)÷2

=(1-1/11)÷2

=10/11÷2

=5/11

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