设等比数列的公比是q
则 a1(1+q³)=28 -------(1)
a1(q+q²)=12 -------(2)
∴ q≠-1
(1)/(2)
(1+q³)/(q+q²)=7/3
∴ (1-q+q²)/q=7/3
∴ 3(q²-q+1)=7q
3q²-10q+3=0
(q-3)(3q-1)=0
∴ q=3或q=1/3
① q=3,则a1=1,an=3^(n-1)
② q=1/3,则a1=27,an=27*(1/3)^(n-1)
设等比数列的公比是q
则 a1(1+q³)=28 -------(1)
a1(q+q²)=12 -------(2)
∴ q≠-1
(1)/(2)
(1+q³)/(q+q²)=7/3
∴ (1-q+q²)/q=7/3
∴ 3(q²-q+1)=7q
3q²-10q+3=0
(q-3)(3q-1)=0
∴ q=3或q=1/3
① q=3,则a1=1,an=3^(n-1)
② q=1/3,则a1=27,an=27*(1/3)^(n-1)