Ordinary derivative
High school
Consider a function $f$ of one variable. The derivative of $f$ is
\[f^{\prime}(x) = \lim_{h \to 0} \frac{f(x + h) - f(x)}{h}:\]the slope of the graph of $f$ at $x$, or the rate at which $f$ changes.
Examples
Worked examples
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$f(t, x) = t x^2$
: None
Undergraduate
The function $f(t, x) = t x^2$ has no ordinary derivative, because it has two variables.