GokyoShumyoSection5Problem20/Solution

Black start  

Black must start by cutting. avoids ko in the corners (see further down), but is a mistake

( at )  

White could play at to avoid further disaster, but then black captures, saving his stones. After white defends, prevents an eye in the corner and white is in trouble.

Oiotoshi  

White needs to fill four times, but black only needs two moves to approach, so white cannot avoid the oiotoshi after .

White can get ko  

White can get ko, however, by playing like this and throwing in with .