设Sn为等差数列{an}的前n项和,若S8=30,S4=7,则a4的值等于(  ) A.14 B.94 C.134 D.174

问题描述:

设Sn为等差数列{an}的前n项和,若S8=30,S4=7,则a4的值等于(  )
A.

1
4

B.
9
4

C.
13
4

D.
17
4

S8=8a1+

8×7
2
d=30①,S4=4a1+
4×3
2
d=7

①②联立可得a1
1
4
,d=1

a4a1+3d=
13
4

故选C