In the function $P(s) = 3s$, the independent variable (input) is the side length $s$ of the equilateral triangle, as it can be chosen freely, and it determines the outcome of the function. The dependent variable (output) is the perimeter $P$, as it depends on the value of $s$.

In summary:

• Independent variable (input): $s$ (side length of the triangle)
• Dependent variable (output): $P$ (perimeter of the triangle)

Would you like additional explanations, or have questions on function variables?

Here are some related questions to expand understanding:

1. What happens to the perimeter if we double the side length $s$?
2. How would you describe a dependent variable in other contexts?
3. If $s = 5$, what is the perimeter of the triangle?
4. Can the function $P(s) = 3s$ apply to non-equilateral triangles?
5. How would you write a similar function for the area of an equilateral triangle?

Tip: Independent variables are often chosen freely, while dependent variables rely on the independent ones in equations.