分子是平方数分母大1(n+1)2/[(n+1)2+1]即(n+1)2/(n2+2n+2)分母不等于0x^2-4x-5=(x+1)(x-5)≠0所以x≠-1且x≠5
=1/3*(1/1*2*3-1/2*3*4)+1/3*(1/2*3*4-1/3*4*5)+……+1/3*(1/17*18*-19-1/18*19*20)=1/3*(1/1*2*3-1/18*19*20)=1139/20520
