1. The **domain** of a function is the set of all possible input values (usually $x$) for which the function is defined. 2. To find the domain, identify values that make the function undefined, such as: - Division by zero (e.g., $\frac{1}{x}$ is undefined at $x=0$). - Square roots (or even roots) of negative numbers in real numbers (e.g., $\sqrt{x}$ requires $x \geq 0$). - Logarithms require positive arguments (e.g., $\log(x)$ requires $x > 0$). 3. Example: For $f(x) = \frac{1}{x-3}$, the domain excludes $x=3$ because it makes the denominator zero. 4. So, the domain is all real numbers except $x=3$, written as $(-\infty, 3) \cup (3, \infty)$. 5. Always check the function's formula for restrictions to determine the domain. Understanding domain helps ensure you only use inputs that produce valid outputs.