a2+a4-(a1+a3)＝12-8 2d=4 d=2
a1+a1+2d=8 a1=2
an=a1+(n-1)d=2n
a1,ak,Sk＋2等比
Sn=na1+n(n-1)d/2=n²+n
ak²=a1*S(k+2)
(2k)²=2*[(k+2)²+k+2]
k=-1(舍去),k=6请问(2k)²=2*[(k+2)²+k+2]是怎么来的？Sn=na1+n(n-1)d/2=n²+nS(k+2)=(k+2)²+(k+2)