Let A, B and x be objects. Assume x∈B. Then we have x∈A∪B.
It is enough to show x∈U({A,B}). We know B∈{A,B}. Using this and x∈B, we are done.
Click here to modify variable names (JavaScript Must Be Enabled).
Show Background.
Test yourself on this item.
Theorem Theorem Theorem Theorem Theorem
