How to understand (CUA) ∩ (cub) = Cu (a ∪ b)

How to understand (CUA) ∩ (cub) = Cu (a ∪ b)

Let me explain it to you in Chinese
(CUA): it means the complement of a in the complete set U;
∩: intersection;
The complement of a in the complete set u is equal to the complement of B in the complete set u after a and B