等比数列an中,a1=3,an=96,Sn=189,则n=?2又1/3+4又1/9+6又1/27....+(2n+1/3的n次方)=
Sn=
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在等比数列{an}中,(1)若Sn=189,q=2,an=96,求a1和n。 (2)若a1+a3=10...答:=(a1-a1q^n)/(1-q)=(a1-an*q)/(1-q)Sn=189,q=2,an=96 189=(a1-96*2)/(1-2)a1=3,an=a1q^(n-1)96=3*2^(n-1)32=2^(n-1)2^5=2^(n-1)n-1=5 n=6 (2) a1+a3=10, a4+a6=5/4 a1(1+q^2)=10(1) , a4(1+q^2)=5/4(2)(2)/(1),得 a4/...