a1 =1,a n+1 =3a n ,则a n =3a n-1 =3*3a n-2=32a n-2=33a n-3=……=3n-1a1=3n-1
问3n-1a1是怎么来的?
解：因为an+1/an=3
an是以a1=1,q=3的等比数列的通项公式
an=a1q=3^n-1
故an =3an-1 =3*3an-2=32an-2=33an-3=……=3n-1a1=3n-1
an是以a1=1,q=3的等比数列的通项公式
an=a1q=3^n-1
故an =3an-1 =3*3an-2=32an-2=33an-3=……=3n-1a1=3n-1
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