42*（1/8+1/24+1/48+1/80+1/120+1/168）=42*1/2*（1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10+1/10-1/12+1/12-1/14）=21*（1/2-1/14）=21*3/7=9
