1/2*3+1/3*4+1/4*5+1/5*6=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+1/5-1/6=1/2-1/6=1/3
1/(2×3)+1/(3×4)+1/(4×5)+1/(5×6)=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6=1/2-1/6=1/3
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6=1/2-1/6=1/3