
1乘以3分之1加3乘以5分之1+5乘以7分之1+......49乘以51分之1
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=1/2×(1-1/3)+1/2×(1/3-1/5)+……+1/2×(1/49-1/51)
=1/2×(1-1/3+1/3-1/5+……+1/49-1/51)
==1/2×(1-1/51)
=25/51
=1/2×(1-1/3+1/3-1/5+……+1/49-1/51)
==1/2×(1-1/51)
=25/51
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