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Bộ 23 đề thi thử THPTQG năm 2018 - Môn Toán - Gv Đặng Việt Đông - Lovebook - Có lời giải chi tiết

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( ) ( )<br />

2 2<br />

2<br />

( 2 1)<br />

2n<br />

1 1 . 2n<br />

1<br />

f n − − + + ( 2n<br />

− 1)<br />

+ 1<br />

Do đó =<br />

<br />

=<br />

2 2<br />

2 .<br />

f ( 2n)<br />

( 2n) + 1 . ( 2n+ 1)<br />

+ 1 ( 2n<br />

+ 1)<br />

+ 1<br />

<br />

2 2 2<br />

2<br />

f ( 1)<br />

f ( 3)<br />

f ( 5)<br />

f ( 2n −1)<br />

1 + 1 3 + 1 5 + 1 ( 2n<br />

− 1)<br />

+ 1<br />

Suy ra un<br />

= . . ... = . . ...<br />

2 2 2<br />

2<br />

f ( 2)<br />

f ( 4)<br />

f ( 6)<br />

f ( 2n)<br />

3 + 1 5 + 1 7 + 1 2n<br />

+ 1 + 1<br />

2 1<br />

u n<br />

= =<br />

2 + 2n+<br />

1<br />

( 2n<br />

+ 1) 2 n<br />

2<br />

n<br />

1 1<br />

lim n un<br />

= lim = = .<br />

2<br />

2n<br />

+ 2n+<br />

1 1 1 2<br />

2 + +<br />

2<br />

n n<br />

( )

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