1又1/2*5+3又1/5*8+5又1/8*11+7又1/11*14+9又1/14*17+11又1/17*20 =1+3+5+7+9+11+1/2*5+1/5*8+.+1/17*20=(1+11)*6/2+1/2*5+1/5*8+.+1/17*20=36+(1/2-1/5+1/5-1/8+...+1/17-1/20)/3=36+(1/2-1/20)/3=36+3/20=36又3/20...