4/1+12/1+24/1+40/1+.19800/1
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4/1+12/1+24/1+40/1+.19800/1
4/1+12/1+24/1+40/1+.19800/1
4/1+12/1+24/1+40/1+.19800/1
4/1+12/1+24/1+40/1+.19800/1
=1/2×(1/2+1/6+1/12+1/20+.+1/9900)
=1/2×(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+.+1/99-1/100)
=1/2×(1-1/100)
=1/2×99/100
=99/200
1/2*(1/2+1/6+1/12+1/20+```+1/9900)=1/2*(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+```+1/99-1/100)=1/2*(1-1/100)=99/200
孩纸···你确定你没有把分数写倒过来么、、、、