(2n-1)/[(n-1)n(n+1)]=1/[n(n+1)]+1/[(n-1)(n+1)]1/[n(n+1)]=1/n-1/(n+1)1/[(n-1)(n+1)]=1/2*[1/(n-1)-1/(n+1)] 1155*(5/2*3*4+7/3*4*5+…+17/8*9*10+19/9*10*11)=1155*(1/3*4+1/1*3+1/4*5+1/3*5+.+1/9*10+1/8*10+1/...