=(1+2+3+4+5+6)+(1/2×3+1/3×4+1/4×5+1/5×6+1/6×7+1/7×8)
=21+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8
=21+1/2-1/8
=21又8分之3