- (1)已知等差数列an中,a1=
- (1)已知等差数列an中,a1=-2,a2+a8=16,求其前11项的和s11.(2)已知等比数列an的公比q不等于1，sn为其前n 项的和，若a1+an=66,a2an-1=128,sn=126,求n和q
- 1)a2+a8=a1+d+a1+7d=2a1+8d=16
8d=16+4
d=5/2
s11=11*(-2)+(11*10/2)*5/2=231/2
2)a1+an=a1+a1*q^(n-1)=66
a2a(n-1)=a1q*a1q^(n-2)=(a1^2)*(q^(n-1))=128
Sn=a1(1-q^n)/(1-q)=126