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
答
∵S8=8a1+
d=30①,S4=4a1+8×7 2
d=7②4×3 2
①②联立可得a1=
,d=11 4
∴a4=a1+3d=
13 4
故选C