蛇Sn=1/2+1/6+1/12+....+1/n(n+1),且Sn*S(n+1)=3/4,则 n的值为A 9 B 8 C 7 D 6
选D：6Sn = 1/2+1/6+1/12+....+1/n(n+1) = 1 -1/(n+1) = n/(n+1)S(n+1) = (n+1)/(n+2)Sn*S(n+1) = 3/4 = [n/(n+1)]*[(n+1)/(n+2)]== n = 6
