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# Evaluate the value of the following expression $\int{\dfrac{\cos 2x-\cos 2\theta }{\cos x-\cos \theta }}dx$.(A) $2\left( \sin x+x\cos \theta \right)+C$ (B) $2\left( \sin x-x\cos \theta \right)+C$(C) $2\left( \sin x+2x\cos \theta \right)+C$(D) $2\left( \sin x-2x\cos \theta \right)+C$

Last updated date: 22nd Mar 2023
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Hint: Use trigonometric identity given as $\cos 2\theta =2{{\cos }^{2}}\theta -1$. Please note that $\cos 2\theta$ and $\cos \theta$are constants.

Here, we have integration given;
$\int{\dfrac{\cos 2x-\cos 2\theta }{\cos x-\cos \theta }}dx$……………………(1)
As, we can notice that ‘x’ is acting as a variable and $\theta$ is a constant as integration is given with respect to dx.
We can use trigonometric identity $\cos 2\theta =2{{\cos }^{2}}\theta -1$ to simplify the integration given in equation (1).
Hence, replacing $\cos 2x\ \text{by }2{{\cos }^{2}}x-1\ \text{and }\cos 2\theta \text{ by }2{{\cos }^{2}}\theta -1$ by using the above mentioned trigonometric identity.
Hence, equation (1) or given integration can be written as;
\begin{align} & \int{\dfrac{\left( 2{{\cos }^{2}}x-1 \right)-\left( 2{{\cos }^{2}}\theta -1 \right)}{\cos x-\cos \theta }}dx \\ & \int{\dfrac{2{{\cos }^{2}}x-1-2{{\cos }^{2}}\theta +1}{\cos x-\cos \theta }}dx \\ & \int{\dfrac{2\left( {{\cos }^{2}}x-{{\cos }^{2}}\theta \right)}{\cos x-\cos \theta }}dx..................\left( 2 \right) \\ \end{align}
Now, we have an algebraic identity given as;
${{a}^{2}}-{{b}^{2}}=\left( a-b \right)\left( a+b \right)$ ………………….(3)
Now, we can use the above algebraic identity as mentioned in equation (3) with equation (2). We can replace ${{\cos }^{2}}x-{{\cos }^{2}}\theta \text{ by }\left( \cos x-\cos \theta \right)\left( \cos x+\cos \theta \right)$.
Hence, equation (2) can be written as;
$\int{\dfrac{2\left( \cos x-\cos \theta \right)\left( \cos x+\cos \theta \right)}{\left( \cos x-\cos \theta \right)}}dx$
Now, cancelling same times from numerator and denominator, we get;
$2\int{\left( \cos x-\cos \theta \right)dx}$ …………………….(4)
Now, we know that;
$\int{\left( f\left( x \right)+g\left( x \right) \right)dx=\int{f\left( x \right)dx+\int{g\left( x \right)dx}}}$
Hence, we can write equation (4), as
$2\int{\cos xdx}+2\int{\cos \theta dx}$
We know integration of cos x is sin x and $\cos \theta$ is acting as a constant with respect to ‘x’. Hence, we can take it out of integration in the above equation, we get;
\begin{align} & 2\sin x+2\cos \theta \int{1dx} \\ & or \\ & 2\sin x+2\cos \theta \int{{{x}^{0}}dx} \\ \end{align}
As we know $\int{{{x}^{n}}dx=\dfrac{{{x}^{n+1}}}{n+1}}$, Hence $\int{{{x}^{0}}dx=\dfrac{{{x}^{0+1}}}{0+1}}=x$
Therefore, above equation can be written as;
\begin{align} & 2\sin x+2\cos \theta \left( x \right)+C \\ & or \\ & 2\left( \sin x+x\cos \theta \right)+C \\ & Hence, \\ & \int{\dfrac{\cos 2x-\cos 2\theta }{\cos x-\cos \theta }}=2\left( \sin x+x\cos \theta \right)+C \\ \end{align}
So, option (A) is the correct answer.
Note: One can apply the trigonometric identity of $\cos 2\theta \text{ as }1-2{{\cos }^{2}}\theta$ which will give the wrong solution. Hence, use $\cos 2\theta =2{{\cos }^{2}}\theta -1$ .
Another approach for this question would be that we can multiply by $\cos x+\cos \theta$ in numerator and denominator, we get;
$\int{\dfrac{\left( \cos 2x-\cos 2\theta \right)\left( \cos x+\cos \theta \right)}{\left( \cos x-\cos \theta \right)\left( \cos x+\cos \theta \right)}}dx$
Using relation $\left( a-b \right)\left( a+b \right)={{a}^{2}}-{{b}^{2}}$, and solve accordingly.