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# How do you find the derivative of $y=\dfrac{{{x}^{2}}-1}{{{x}^{2}}+1}$?

Last updated date: 24th Jul 2024
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Hint: To solve this problem, we should know the derivatives of some of the functions. We should also know the quotient rule of differentiation which is used to differentiate expressions of form $\dfrac{f(x)}{g(x)}$. The function whose derivatives we should know is ${{x}^{2}}$, its derivative with respect to x is $2x$. The quotient rule states that the expressions of the form $\dfrac{f(x)}{g(x)}$ are differentiated as,
$\dfrac{d\left( \dfrac{f(x)}{g(x)} \right)}{dx}=\dfrac{f'\left( x \right)g\left( x \right)-g'\left( x \right)f\left( x \right)}{{{\left( g\left( x \right) \right)}^{2}}}$.

Complete step by step solution:
We are given the expression $y=\dfrac{{{x}^{2}}-1}{{{x}^{2}}+1}$, we need to find its derivative. This expression is of the form $\dfrac{f(x)}{g(x)}$. Here, $f(x)={{x}^{2}}-1\And g(x)={{x}^{2}}+1$. We will use the quotient rule to differentiate this expression. We know that the quotient rule states that expressions of the form $\dfrac{f(x)}{g(x)}$ are differentiated as, $\dfrac{d\left( \dfrac{f(x)}{g(x)} \right)}{dx}=\dfrac{f'\left( x \right)g\left( x \right)-g'\left( x \right)f\left( x \right)}{{{\left( g\left( x \right) \right)}^{2}}}$.
We know that the derivative of ${{x}^{2}}$ with respect to x is $2x$. Thus, the derivative of ${{x}^{2}}-1$ with respect to x is also $2x$. Similarly, the derivative of ${{x}^{2}}+1$ with respect to x is also $2x$.
Using the quotient rule on the expression $y=\dfrac{{{x}^{2}}-1}{{{x}^{2}}+1}$, we get
$\dfrac{d\left( y \right)}{dx}=\dfrac{\dfrac{d\left( {{x}^{2}}-1 \right)}{dx}\left( {{x}^{2}}+1 \right)-\dfrac{d\left( {{x}^{2}}+1 \right)}{dx}\left( {{x}^{2}}-1 \right)}{{{\left( {{x}^{2}}+1 \right)}^{2}}}$
Substituting the derivatives of the functions, we get
$\dfrac{dy}{dx}=\dfrac{2x\left( {{x}^{2}}+1 \right)-2x\left( {{x}^{2}}-1 \right)}{{{\left( {{x}^{2}}+1 \right)}^{2}}}$
Expanding the brackets, we get
$\dfrac{dy}{dx}=\dfrac{2{{x}^{3}}+2x-2{{x}^{3}}+2x}{{{\left( {{x}^{2}}+1 \right)}^{2}}}$
Simplifying the above expression, we get
$\Rightarrow \dfrac{dy}{dx}=\dfrac{4x}{{{\left( {{x}^{2}}+1 \right)}^{2}}}$
Thus, the derivative of the expression $y=\dfrac{{{x}^{2}}-1}{{{x}^{2}}+1}$ is $\dfrac{dy}{dx}=\dfrac{4x}{{{\left( {{x}^{2}}+1 \right)}^{2}}}$.

Note:
To solve these questions, one should know the derivatives of the functions and quotient rules. We can also solve the differentiate the expression as,
$y=\dfrac{{{x}^{2}}-1}{{{x}^{2}}+1}$
Adding and subtracting 1 in the numerator, we get
$\Rightarrow y=\dfrac{\left( {{x}^{2}}+1 \right)+\left( -1-1 \right)}{{{x}^{2}}+1}$
$\Rightarrow y=\dfrac{{{x}^{2}}+1}{{{x}^{2}}+1}-\dfrac{2}{{{x}^{2}}+1}$
$\Rightarrow y=1-\dfrac{2}{{{x}^{2}}+1}$
Differentiating both sides of the above expression, we get
$\Rightarrow \dfrac{dy}{dx}=\dfrac{d(1)}{dx}-\dfrac{d\left( \dfrac{2}{{{x}^{2}}+1} \right)}{dx}$
$\Rightarrow \dfrac{dy}{dx}=0+\dfrac{2\times 2x}{{{\left( {{x}^{2}}+1 \right)}^{2}}}=\dfrac{4x}{{{\left( {{x}^{2}}+1 \right)}^{2}}}$
Thus, we are getting the same answer from both methods.