# 4: Applications of the Derivative


• 4.1: Interpretating, Estimating, and Using the Derivative
Regardless of the context of a given function $$y = f (x)$$, the derivative always measures the instantaneous rate of change of the output variable with respect to the input variable. The units on the derivative function $$y = f'(x)$$ are units of $$f$$ per unit of $$x$$. Again, this measures how fast the output of the function $$f$$ changes when the input of the function changes.
• 4.2: The Second Derivative
A differentiable function f is increasing at a point or on an interval whenever its first derivative is positive, and decreasing whenever its first derivative is negative. By taking the derivative of the derivative of a function f', we arrive at the second derivative, f''. The second derivative measures the instantaneous rate of change of the first derivative, and thus the sign of the second derivative tells us whether or not the slope of the tangent line to f is increasing or decreasing.
• 4.3: The Tangent Line Approximation
The principle of local linearity tells us that if we zoom in on a point where a function y = f (x) is differentiable, the function should become indistinguishable from its tangent line. That is, a differentiable function looks linear when viewed up close.
• 4.4: Elementary Derivative Rules
The limit definition of the derivative leads to patterns among certain families of functions that enable us to compute derivative formulas without resorting directly to the limit definition. If we are given a constant multiple of a function whose derivative we know, or a sum of functions whose derivatives we know, the Constant Multiple and Sum Rules make it straightforward to compute the derivative of the overall function.

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