1/(2×3)=1/6，那么 1/2-1/3=1/6∴1/(2×3)=1/2-1/31/(3×4)=1/3-1/41/(4×5)=1/4-1/51/(5×6)=1/5-1/61/(6×7)=1/6-1/7.........1/[a×(a+1)]=1/a-1/(a+1)
