1/(1×2)=1-1/21/(2×3)=1/2-1/31/(3×4)=1/3-1/41/(4×5)=1/4-1/51/(5×6)=1/5-1/6 1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5)+1/(5×6)=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6=1-1/6=5/6
