设数列{an}是首项为a1公差为-2的等差数列,如果a1+a4+a7=50,那么a3+a6+a9=(  ) A.28 B.-78 C.-48 D.38

问题描述:

设数列{an}是首项为a1公差为-2的等差数列,如果a1+a4+a7=50,那么a3+a6+a9=(  )
A. 28
B. -78
C. -48
D. 38

∵a1+a4+a7 =50,∴3a1+

3×2
2
×(−6)=50,∴3a1=68,
∴a3+a6+a9 =3(a1-4)+
3×2
2
×(−6)
=3a1-12-18=38,
故选D.