1/(1*2*3)+1/(2*3*4)+1/(3*4*5)+.+1/[n(n+)(n+)]
=1/2(1/1*2-1/2*3)+1/2(1/2*3-1/3*4)+……+1/2[1/n(n+1)-1/(n+1)(n+2)]
=1/2[(1/1*2-1/2*3)+(1/2*3-1/3*4)+……+1/n(n+1)-1/(n+1)(n+2)]
=1/2[1/2-1/(n+1)(n+2)]
=(n^2+3n)/(4n^2+12n+8)
2.已知m=（x^2+2x+1)(x^2-2x+1),n=(x^2+x+1)(x^2-x+1),试确定m与n的大小关系
m=(x^2+1)^2-4x^2
n=(x^2+1)^2-x^2
m-n=-3x^2