I know how to formulate the dual problem to a standard minimisation Linear Program. However I came across this example which confused me slightly.
Write down the dual problem of:
The answer to this is:
It is just the last line of the solution that confuses me, I assumed it was always $y_1\ge0, y_2\ge0$ so this would become $-y_1\le0, -y_2\le0$