chunhui2002
幼苗
共回答了10个问题采纳率:100% 举报
数列2*5分之一,5*8分之一,8*11分之一,…,(3n-2)(3n+2)分之一,…的前n项和为(B)
A n/(3n+2)
B n/(6n+4)
C 3n/(6n+4)
D (n+1)/(n+2)
2*5分之一+5*8分之一+8*11分之一+…+(3n-2)(3n+2)分之一
=1/3*[1/2-1/5+1/5-1/8+1/8-1/11+…+1/(3n-2)-1/(3n+2)]
=1/3*[1/2-1/(3n+2)]
=1/3*3n/2(3n+2)
=n/(6n+4)
1年前
8