(1){an}为等比数列a1=7,a4=a1q^3=-56,
∴q=-2
∴a5=a1q^4=56,
sn=a1(1-q^n)/(1-q)=7[1-(-2)^n]/3
∴s5=67
(2)an=a1q^(n-1)=96,①
sn=a1(1-q^n)/(1-q)=189②
解方程组得:n= 6,q= 2
(1){an}为等比数列a1=7,a4=a1q^3=-56,
∴q=-2
∴a5=a1q^4=56,
sn=a1(1-q^n)/(1-q)=7[1-(-2)^n]/3
∴s5=67
(2)an=a1q^(n-1)=96,①
sn=a1(1-q^n)/(1-q)=189②
解方程组得:n= 6,q= 2