Question

# Find the derivative of x at x = 1.

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Hint- Try to solve using the definition of derivative i.e. $f\prime \left( x \right) = \mathop {\lim }\limits_{h \to 0} \dfrac{{f\left( {x + h} \right) - f\left( x \right)}}{h}$

Let $f\left( x \right) = x$
We need to find the derivative of $f\left( x \right)$at${\text{x = 1}}$.
${\text{i}}{\text{.e}}{\text{. }}f\prime \left( 1 \right)$
We know that,
$f\prime \left( x \right) = \mathop {\lim }\limits_{h \to 0} \dfrac{{f\left( {x + h} \right) - f\left( x \right)}}{h}{\text{ }} \ldots \left( 1 \right)$
Here, $f\left( x \right) = x$
So, ${\text{ }}f\left( {x + h} \right) = x + h$
Putting these values in equation $\left( 1 \right)$, we get
$f\prime \left( x \right) = \mathop {\lim }\limits_{h \to 0} \dfrac{{\left( {x + h} \right) - x}}{h} \\ = \mathop {\lim }\limits_{h \to 0} \dfrac{{x + h - x}}{h} \\ = \mathop {\lim }\limits_{h \to 0} {\text{ }}\dfrac{h}{h} \\ = \mathop {\lim }\limits_{h \to 0} {\text{ }}1 \\ = 1 \\$
Hence, $f\prime \left( x \right) = 1$
Putting $x = 1$, we get
$f\prime \left( 1 \right) = 1$
Hence, the derivative of $x$ at $x=1$ is $1$.

Note- In order to calculate the derivative of a certain function, we assume that function to be $f\left( x \right)$ and apply the formula of the derivative and later put the value of $x$ for which the derivative is asked to calculate.