2: Derivatives
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The derivative of a function f is another function, f′, defined as f′(x)≡dfdx≡limδx→0f(x+δx)−f(x)δx. This kind of expression is called a limit expression because it involves a limit (in this case, the limit where δx goes to zero).
If the derivative exists within some domain of x (i.e., the above limit expression is mathematically well-defined), then we say f is differentiable in that domain. It can be shown that a differentiable function is automatically continuous.
Graphically, the derivative represents the slope of the graph of f(x), as shown below:

If f is differentiable, we can define its second-order derivative f″ as the derivative of f′. Third-order and higher-order derivatives are defined similarly.
- 2.4: Partial Derivatives
- Functions can also take multiple inputs; for instance, a function f(x,y) maps two input numbers, x and y , and outputs a number. In general, the inputs are allowed to vary independently of one another. The partial derivative of such a function is its derivative with respect to one of its inputs, keeping the others fixed.