由题知,已知公差不为零的等差数列{an}中a1,a2,a5,成等比数列.设公差为da2²=a1*a5所以,(a1+d)²=a1*(a1+4d)展开得a1²+2a1*d+d²=a1²+4a1*d所以,2a1=d所以,a1:a2:a5=1:3:9所以,等比数列公比为32)若等差数列an的首相a1=1d=2a1=2所以,sn=na1+(n-1)nd/2
由题知,已知公差不为零的等差数列{an}中a1,a2,a5,成等比数列.设公差为da2²=a1*a5所以,(a1+d)²=a1*(a1+4d)展开得a1²+2a1*d+d²=a1²+4a1*d所以,2a1=d所以,a1:a2:a5=1:3:9所以,等比数列公比为32)若等差数列an的首相a1=1d=2a1=2所以,sn=na1+(n-1)nd/2