6+1/42+7+1/56+8+1/72.+20+1/420=6+7+8+...+20+1/42+1/56+1/72+...+1/420=[(6+20)*7+13]+(1/6*7)+(1/7*8)+(1/8*9)+...+(1/20*21)=195+1/6-1/7+1/7-1/8+1/8-1/9+...+1/20-1/21=195+1/6-1/21=195+7/42-2/42=195+5/42=1...