先算除法后相加
1+1／3+1／6+1／10+1／15+1／21+1／28
=1+2*(1/6+1/12+1/20+1/30+1/42+1/56)
=1+2*(1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+1/7*8)
=1+2*(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8)
=1+2*(1/2-1/8)
=1+2*3/8
=1+3/4=7/4
原式=1+1/3（1+1/2）+1/5（1/2+1/3）+1/7(1/3+1/4)
=1+1/2+1/6+1/12
=7/4
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