1+2+3+……+(n-1)+n=(1+n)x(n/2)=n(n+1)/21+2+3+……+(n-2)+(n-1)=[1+(n-1)]x(n-1)/2=n(n-1)/21/n+2/n+.+(n-1)/n=[n(n-1)/2])/n=(n-1)/2所以1/2+1/3+2/3+1/4+2/4+3/4+1/5+2/5+…+1/50+2/50…+49/50=(2-1)/2+(3-1)/2+...