第1个回答 2009-05-21
s1=a1
s2=a1+a2
s4=a1+a2+a3+a4
s2^2=s1*s4
a1^2(1+q)^2=a1^2(1+q+q^2+q^3)
(1+q)^2=[(1+q)(1+q^2)]
(1+q)(1+q+1+q^2)=0
q=-1或q1=...
第2个回答 2009-05-21
1,公比为2,S1=a,S2=2a,S4=4a
a=3,an=3n
2,q=2
第3个回答 2009-05-21
2
3*6(n-1)
2
第4个回答 2009-05-21
设等差数列第一项为a1,公差为d
S1=a1
S2=2a1+d
S4=4a1+6d
S1,S2,S4成等比数列,即
(2a1+d)^2=a1(4a1+6d)
解出d=2a1
q=S2/S1=(2a1+d)/a1=4
若S2=6,则a1=1.5,d=3
an=1.5+3(n-1)=3n-1.5
2、
设公差为d
a2=a1+d
a4=a1+3d
a1,a2,a4成等比数列,即
(a1+d)^2=a1(a1+3d)
解出d=a1
q=a2/a1=(a1+d)/a1=2
第5个回答 2009-05-21
S1=a1
S2=a1+a2=a1+a1+d=2a1+d
S4=a1+a2+a3+a4=4a1+(1+2+3)d=4a1+6d
S2^2=S1×s4
(2a1+d)^2=a1×(4a1+6d)
a1×q=2a1+d
a1×q^2=4a1+6d
[(2a1+d)÷a1]^2=(4a1+6d)÷a1
4a1^2+4a1d+d^2=4a1^2+6a1d
2a1d-d^2=0 即2a1=d
S2=2a1+d=2d=6 d=3 a1=3/2
等差数列的通项公式 3n-3/2
q=(2a1+d)/a1=4
2 a2^2=a1×a4
(a1+d)^2=a1×(a1+3d) a1=d
q=a2/a1=(a1+d)/a1=(2a1)/a1=2