已知等差数列【an]中.a7+a9=16,a4=1.则an12等于是a12等于多少_数学解答

已知等差数列【an]中.a7+a9=16,a4=1.则an12等于
是a12等于多少

人气：232 ℃　时间：2020-05-05 22:12:10

解答

等差数列
a7,a9,等差中项是 a8
a7 + a9 = 2a8
a4,a12,等差中项也是 a8
a4 + a12 = 2 a8 = a7+ a9 = 16
a12 = 16 -a4 = 16 -1 = 15