The best way to understand a set is to draw a picture. Think of A and B as two different ○ Think of U as a big rectangle (and this big rectangle is the largest one containing A and B). "Cu" means to exclude the letter immediately after U, and CuA is the figure that cuts out the circle A from the big rectangle.

"∨" means merging the two parts, so A ∨( CuA)= U means merging the circle of A with the figure in the big rectangle, and the result is of course a U- big rectangle.

"∩" is the intersection, and the result he represents is the overlapping part of the graphics represented by the letters on both sides of the symbol.

So A∩(CuA) has no overlapping parts, so the result is "φ" empty sets-nothing.

In the same way, Cu(CuA)=A excludes "the figure with the circle A removed" from the big rectangle, and the result is A itself.

The formula Cu(CuB)=(CuA)∨( CuB) is incorrect.

It should be Cu(A∩B)=(CuA)∩( CuB), which means the figure excluding the overlapping part of A and B in the big rectangle = the figure excluding A in the big rectangle ∪ the figure excluding B in the big rectangle.

Cu(A∪B)=(CuA)∩(CuB) means the figure that does not include the merged part of A and B in the big rectangle = the figure that does not include A in the big rectangle ∩ the figure that does not include B in the big rectangle.