Associative law (Sets)

The associative law says that $(A\cup B)\cup C=A\cup (B\cup C)$ and that $(A\cap B)\cap C=A\cap (B\cap C)$.

We can meaningfully write, $A\cup B\cup C$ or ${\bigcup }_{i=1}^n{A}_i$.

Similarly, we can also write, $A\cap B\cap C$ or ${\bigcap }_{i=1}^n{A}_i$.