42×(1/8+1/24+1/48+1/80+1/120+1/168)
=42×1/4×(1/2+1/6+1/12+1/20+1/明伏厅30+1/42)
=42×1/4×(1/1*2+1/2*3+1/3*4+1/4*5+1/5*6+1/6*7)
=42×1/4×(1-1/2+1/2-1/3+1/3-1/4+1/4-1/厅李5+1/5-1/6+1/6-1/激隐7)
=42×1/4×(1-1/7)
=42×1/4×6/7
=9
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