2.等差数列的性质 (1)若{an}是公差为d的等差数列,则下列数列: ①{c+an}(c为任一常数)是公差为 的等差数列; ②{c·an}(c为任一常数)是公差为 的等差数列; ③{an+an+k}(k为常数,k∈N+)是公差为 的等差数列. (2)若{an},{bn}分别是公差为d1,d2的等差数列,则数列{pan+qbn}(p,q是常数)是公差为 的等差数列. (3)若{an}为等差数列,m,k,n成等差数列,则am,ak,an也成 (m、k、n∈N*),即若m+n=2k,则 . (4)在等差数列{an}中,下标成等差数列且公差为m的项, ak,ak+m,ak+2m,…(k,m∈N*)仍然组成等差数列.
cd
2d
pd1+qd2
等差数列
am+an=2ak