No.680(Answers)

Black 1 is crucial.
The result is a Ko with the moves to White 6.

Black 1 is crucial.
If White 2, Black 3 is the vital point.
If White 4 and 6, Black gets a Ko with the moves to 15.(one-move approach Ko for Black)

If White avoids the Ko with 4 and 6, Black lives with 7 to 17 in a Seki.