So a=- 1 or 0 b equals-1 or 0.

Because a≤b, there are three situations.

①: when a=- 1 b=- 1

c= 1

② when a=- 1 b=0.

C=- 1/2 (Yang's calculation of 123 is wrong)

③ When a=0 b=0

c=0

To sum up, the set U={0, 1,-1/2}

Setting a:

- 1