已知等差数列{an}中,a7+a9=16,a4=1,则a12的值是( ) A.15 B.30 C.31 D.64
问题描述:
已知等差数列{an}中,a7+a9=16,a4=1,则a12的值是( )
A. 15
B. 30
C. 31
D. 64
答
解法1:∵{an}为等差数列,设首项为a1,公差为d,∴a7+a9=a1+6d+a1+8d=2a1+14d=16 ①;a4=a1+3d=1 ②;由①-②得a1+11d=15,即a12=15.解法2:由等差数列的性质得,a7+a9=a4+a12,∵a...