在等比数列中,a3=4 S3=12
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a4=a1*q³q³=64q=4S4=a1+a2+a3+a4=1+4+16+64=85——————————————————S3=2/9即a1+a2+a3=2/9a3/q²
a3=2S2+1a4=2S3+1两式相减,得a4-a3=2(S3-S2)=2a3即,a4=3a3则,q=a4/a3=3所以,公比是3再问:S3-S2=a3?再答:对S3=a1+a2+a3S2=a1+a
a4-a3=2s3+1-(2s2+1)=2s3-2s2=2a3a4=2a3+a3=3a3q=3
S3=a1+a2+a3=a1+a1q+a1q^2=a1(1+q+q^2)=9/2(1)a3=a1q^2=3/2(2)(1)/(2)2q^2-q-1=0(q-1)(q+1/2)=0q=1或q=-1/2当
S3=a3+a1+a2=a3/q^2+a3/q+a3所以9/2=3/2(1+1/q^2+1/q)解得q=1或q=-1/2q=1时,a1=3/2;q=-1/2时,a1=6
等比数列{an}中,∵a3=2S2+1,a4=2S3+1,∴a4-a3=2S3+1-(2S2+1)=2(S3-S2)=2a3,∴a4=3a3,∴q=a4a3=3.故选:C.
a1=2,a2=a1q=2qa3=a1q^2=2q^2S3=26a1+a2+a3=s32+2q+2q^2=26q^2+q-12=0(q+4)(q-3)=0q=-4,a3=a1q^2=2q^2=32q=
1式:S3=〔a1(1-q3)〕/(1-q)=42式:S6=〔a1(1-q6)〕/(1-q)=36然后1式:2式得,1/(1+q3)=1/9q=2a1=4/7an=a1*q(n-1)=(自己算吧)
等差6a2=a1+3+a3+4=a1+a3+7a1+a3=6a2-7S3=a1+a2+a3=7所以6a2-7+a2=7a2=2所以a1+a3=5即a2/q+a2q=5所以两边乘q2+2q²=
1、S8=S4*q^4=1*16=162、S3=-12+(-12)/q+(-12)/q^2=-93q^2+12q+12=0q^2+4q+4=0q=-2再问:(^)这个是什么再答:乘方的意思
q^2*a1=2a1(q+1)+1q^3*a1=3a1(q^2+q+1)+1联立求解即可如果是a4=2S3+1的话即q^3*a1=2a1(q^2+q+1)+1q^2*a1=2a1(q+1)+1q^3-
a3=3S2+1,a3-3S2=1,a4=2S3+1a4-2S3=1a3-3S2=a4-2S3a3=a4-2S3+3S2a3=a4-2S3+2S2+s2a3=a4-2(S3-S2)+s2a3=a4-2
(1)q=1a1=3/2Sn=na1=3n/2(2)q=-1/2a1=6Sn=a1(1-q^n)/(1-q)=4-4(-1/2)^n
当等比数列{an}的公比q=1时,显然满足题意;当q≠1时,S3=7q2+7q+7=21,解得q=-12,或q=1(舍去)综合可得q=1或-12故答案为:1或-12.
由a4-a3=2S3+1-2S2-1=2(S3-S2)=2a3得a4=3a3,所以q=a4/a3=3
s6=a1+a2+a3+a4+a5+a6=a1+a2+a3+q^3(a1+a2+a3)31.5=3.5+3.5q^33.5q^3=28q^3=8q=2s3=a1+a2+a3=a1+a1q+a1q^2=
A3=S2+1=A1+A2+1A4=S3+1=A1+A2+A3+1A4-A3=A32a3=a4a4/a3=2所以公比q=2
s3=a1(q^2+q+1)=26因为a1=2所以解得q=3所以a3=a1*q^2=18
a1*a3=a2^2=4a2=2a1*q=2(1)s3=a1(1-q^3)/1-q=7(2)(1)÷(2)得2+2q+2q^2=7q得q=1/2舍q=2a1=1a5=a1*q^4=1*2^4=16
S3-S2=(a1+a2+a3)-(a1+a2)=a3所以2(S3-S2)=2a3