Product Rule Of Differentiation

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Suppose y is a product of two functions u(x) and v(x). This means y = u(x) v(x).

Now my task is to differentiate y, that is, to get the value of \cfrac{dy}{dx}. Since y is a product of two functions, I’ll use the product rule of differentiation to get the value of \cfrac{dy}{dx}. Thus \cfrac{dy}{dx} will be

\[\frac{dy}{dx} = u(x)\frac{dv}{dx}+ v(x)\frac{du}{dx}.\]

Next, I will give some example .

EXAMPLE 

According to Stroud and Booth (2013)* “Differentiate y = x^2 \cos^ 2 x.”

SOLUTION

STEP 1

Here the given function is: y = x^2 \cos^2 x.

Now y is a product of two functions x^2 and \cos^2 x.

In order to differentiate y with respect to x, I’ll use the product rule of differentiation.

Thus it will be

\[\frac{dy}{dx} = x^2\frac{d}{dx} (\cos^2 x) + \cos^2 x \frac{d}{dx}(x^2).\]

So this means 

\[\frac{dy}{dx} = x^2 (2 \cos x)\frac{d}{dx}(\cos x) + \cos^2 x (2x).\]

Now this gives 

\[\frac{dy}{dx} = x^2 (2 \cos x)(-\sin x) + 2x \cos^2 x.\]

At the end I’ll simplify it to get the value of \cfrac{dy}{dx} as

\begin{eqnarray*} \frac{dy}{dx} &=& -2 x^2 \cos x \sin x + 2x \cos^2 x\\ &=& 2x \cos x (\cos x - x \sin x). \end{eqnarray*}

Hence I can conclude that this is the answer to the given example.

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