1、下角标成等差数列的项仍是等差数列
是说,如果an是等差数列,a1,a3,a5````a(2n-1)还是等差数列,这个可以扩大点即,an,a(n+m),a(n+2m)``````还是等差数列,很好证明,可以得到公差为md
2、连续m项和构成的数列成等差数列
是说（a1+a2+a3+·····+am), (a(m+1)+a(m+2)+````+a(2m)),(a(2m+1)+a(2m+2)+````+a(3m))`````````还是等差数列,可以把(a(m+1)+a(m+2)+````+a(2m))提出一个md,就和（a1+a2+a3+·····+am)一样了,所以公差也是md