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
答
∵a1+a4+a7 =50,∴3a1+
×(−6)=50,∴3a1=68,3×2 2
∴a3+a6+a9 =3(a1-4)+
×(−6)=3a1-12-18=38,3×2 2
故选D.