I tried to prove this question but without a success: Let $K_1 \text{and } K_2$ be definable sets, prove that $K_1∪K_2$ is definable.
What I tried to do is to assume: $K_1=\text{Ass}(X)=\{v\mid v\models X\}$ and $K_2=\text{Ass}(Y)=\{v\mid v\models Y\}$ and to say that $K1\cup K2=\text{Ass}(X\cap Y)$. But I couldn't prove that: $K_1\cup K_2\supseteq\text{Ass}(X\cap Y)$.
I would like to get help with this proof