Life In 19x19

I've been working through Kaoru Iwamoto's "Go for Beginners", and got myself stuck on one of the life and death examples.

The problem starts like this with white to move in order to kill Black's group:

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[go]$$W Dia. 17
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X . . X O . |
$$ . . . . . . . X . . |
$$----------[/go]

Kaoru states that the only possible move for White to kill Black is:

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[go]$$W Dia. 17a
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X . . X O . |
$$ . . . . 1 . . X . . |
$$----------[/go]

Which is followed by:

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[go]$$W Dia. 17b
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X . 3 X O . |
$$ . . . . 1 2 . X . . |
$$----------[/go]

That makes perfect sense to me. What I don't understand is why this is the only way to kill Black.

Why not this?

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[go]$$W Dia. 17c
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X . 1 X O . |
$$ . . . . . . . X . . |
$$----------[/go]

If black responds by playing where white should have played, then:

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[go]$$W Dia. 17d
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X 3 1 X O . |
$$ . . . . 2 . . X . . |
$$----------[/go]

And isn't the group still dead?

What am I missing?

GammaGoPBR wrote:
Why not this?

$$W Dia. 17c
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X . 1 X O . |
$$ . . . . . . . X . . |
$$----------

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[go]$$W Dia. 17c
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X . 1 X O . |
$$ . . . . . . . X . . |
$$----------[/go]

What am I missing?

What about:

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[go]$$W Dia. 17c
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X 2 1 X O . |
$$ . . . . . . . X . . |
$$----------[/go]

GammaGoPBR wrote: If black responds by playing where white should have played, then:

$$W Dia. 17d
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X 3 1 X O . |
$$ . . . . 2 . . X . . |
$$----------

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[go]$$W Dia. 17d
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X 3 1 X O . |
$$ . . . . 2 . . X . . |
$$----------[/go]

And isn't the group still dead?

What am I missing?

For studying:

In the case you failed (here: to live; with 2), think about the value of the board point that has been occupied by your opponent's initial reply (here: 3). It might be the vital point (of the problem) that you should have taken beforehand.

BTW, what is the story if White plays in this diagram?

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[go]$$W Can Black live?
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X 1 . X O . |
$$ . . . . . . . X . . |
$$ --------------------[/go]

Bill Spight wrote:BTW, what is the story if White plays in this diagram?

$$W Can Black live?
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X 1 . X O . |
$$ . . . . . . . X . . |
$$ --------------------

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[go]$$W Can Black live?
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X . . X O . |
$$ . . . O X 1 . X O . |
$$ . . . . . . . X . . |
$$ --------------------[/go]

Isn't that also seki:

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[go]$$W seki
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X 5 . X O . |
$$ . . . O X 1 2 X O . |
$$ . . . . 4 3 . X . . |
$$ --------------------[/go]

This doesn't work for white:

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[go]$$W black lives
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X 6 . X O . |
$$ . . . O X 1 2 X O . |
$$ . . . . 3 4 5 X . . |
$$ --------------------[/go]

or

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[go]$$W black lives
$$ . . . . . . . . . . |
$$ . . . O O O O O . . |
$$ . . . O X X X X O . |
$$ . . . O X 6 . X O . |
$$ . . . O X 1 2 X O . |
$$ . . . 5 3 4 . X . . |
$$ --------------------[/go]