1. Let's clarify the problem: You want to know where unknown variables typically appear in mathematical expressions or equations. 2. Unknown variables are usually represented by letters such as $x$, $y$, or $z$ in algebraic expressions. 3. They can appear anywhere in an equation: on the left side, right side, or both. 4. For example, in the equation $2x + 3 = 7$, $x$ is the unknown variable. 5. The goal is to find the value of the unknown variable that makes the equation true. 6. Variables can also appear in functions, inequalities, and systems of equations. 7. Remember, unknown variables are placeholders for values we need to determine.