已知a3=3/2,S3=9/2,求a1与q.
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已知a3=3/2,S3=9/2,求a1与q.
已知a3=3/2,S3=9/2,求a1与q.
已知a3=3/2,S3=9/2,求a1与q.
a3=a1q^2=3/2
S3=a1+a1q+a1q^2=9/2
q^2/(1+q+q^2)=1/3
2q^2-q-1=0
(2q+1)(q-1)=0
q=-1/2, a1=(3/2)/q^2=6
或:q=1, a1=3/2