1. **Problem:** Find the difference between $A$ and $B$, and explain $A$ or $B$. 2. **Formula used:** - The **difference** is $A\setminus B$, which means “in $A$ but not in $B$.” - The **or** of two sets is the **union**, written $A\cup B$, which means “in $A$ or in $B$ or in both.” 3. **Important rule:** - $A\setminus B$ keeps only elements that belong to $A$ and removes anything also in $B$. - $A\cup B$ combines everything from both sets without repeating overlaps. 4. **Using the example sets:** $$A=\{1,2,3\},\quad B=\{3,4,5\}$$ $$A\setminus B=\{1,2\}$$ $$A\cup B=\{1,2,3,4,5\}$$ 5. **Final answer:** - **Difference:** $A\setminus B=\{1,2\}$ - **“A or B”:** $A\cup B=\{1,2,3,4,5\}$