1题...1/n-1/(n+1)=1/[n×(n+1)]; 1-1/(n+1)=n/(n+1)=(1-1/2)+(1/2-1/3)+(1/3-1/4)···+[1/n-1/(n+1)]; 1/(1x2)+1/(2x3)+1/(3x4)+……+1/n(n+1)=1-1/2+1/2-1/3+1/3-1/4···+/n-1/(n+1)=1-1/(n+1)=n/(n+1)
2题··· 2 6 12 20 30 42 56 72 90 10
3题···2/3=1/2+1/6、1=1/2+1/3+1/6、3/5=1/2+1/10,