⑴1/2×3+1/3×4+1/4×5=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)=1/2-1/5=3/10⑵1/3×4+1/4×5+1/5×6+1/6×7=(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/54-1/7)=1/3-1/7=4/21。
