Sn+ 1=4

One; one

+2，Sn=4

an- 1+2

∴an+ 1

=2an-4an- 1, which means an+ 1.

-2an=

2 (Ann

-2an- 1)

hello

=an+ 1

-2an

{bn} Yes.

geometric series

Author S2=

a 1

+

a2=4

a 1

+2 means a2=5.

∴{bn} is a geometric series,

First item

B 1=a2.

-2a 1=

three

(2) According to (1), BN = an+1-2an = 3 * 2n-1,

∴an+ 1/2n+ 1-an/2n=3/4

Make cn=

An/2n represents cn+ 1-cn.

=3/4

∴ series {cn} is the first term of 1/2, and the tolerance is 3/4.

arithmetic series

∴an/2n= 1/2+3/4*(n- 1)=3/4*n- 1/4，

an=(3n- 1)*2n-2