5/6=1/2+1/3,7/12=1/3+1/4,同理可得后面的9/20,11/30,13/42,15/56,17/72.所以原式=1-(1/2+1/3)+1/3+1/4……-(1/6+1/7)+1/7+1/8-(1/8+1/9)=7/18
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用“学生用计算机”算一下就行了。
