Why zero is kept constant in derivative questions?

In derivative questions, the value of zero is often used as a reference point for evaluating the rate of change of a function at a specific point. By setting the change in the variable (Δx) to zero, we can analyze the instantaneous rate of change at that point, which is the essence of the derivative. This approach allows us to find the slope of the tangent line to the curve at a given point, providing important information about the function's behavior.