an=sn-sn-1
=3/2an-1/2-(3/2an-1-1/2)
=3/2an-1/2-3/2an-1+1/2
=3/2an-3/2an-1
æ以,an=3/2an-3/2an-1
æ以,3/2an-1=1/2an
æ以,3an-1=an
æ以,an/an-1=3
æ以,q=3
s1=3/2a1-1/2
æ以,a1=1
æ以,an=a*q^n-1
=1*3^n-1
=3^(n-1)
æ¤æ¶an满足a1=1
æ以,an=3^(n-1)
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