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Indefinite Integrals question

2004 · Shift 0 · Q82
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Indefinite Integrals question

2004 · Shift 0 · Q82

JEE MainMathematicsIndefinite IntegralsMCQ+4 / −1
If ∫sin⁡xsin⁡(x−α)dx=Ax+Blog⁡sin⁡(x−α),+C,\int {{{\sin x} \over {\sin \left( {x - \alpha } \right)}}dx = Ax + B\log \sin \left( {x - \alpha } \right), + C,}∫sin(x−α)sinx​dx=Ax+Blogsin(x−α),+C, then value of (A,B)(A, B)(A,B) is
  1. A
    (−cos⁡α,sin⁡α)\left( { - \cos \alpha ,\sin \alpha } \right)(−cosα,sinα)
  2. B
    (cos⁡α,sin⁡α)\left( { \cos \alpha ,\sin \alpha } \right)(cosα,sinα)
  3. C
    (−sin⁡α,cos⁡α)\left( { - \sin \alpha ,\cos \alpha } \right)(−sinα,cosα)
  4. D
    (sin⁡α,cos⁡α)\left( { \sin \alpha ,\cos \alpha } \right)(sinα,cosα)
View written solutionFree

Correct answer: B

  1. We need to evaluate
intsin⁡xsin⁡(x−α) dx\\int \frac{\sin x}{\sin(x-\alpha)}\,dxintsin(x−α)sinx​dx

and compare it with

Ax+Blog⁡sin⁡(x−α)+C.Ax + B\log \sin(x-\alpha) + C.Ax+Blogsin(x−α)+C.
  1. Express sin⁡x\sin xsinx in terms of (x−α)(x-\alpha)(x−α):
sin⁡x=sin⁡((x−α)+α).\sin x = \sin\big((x-\alpha)+\alpha\big).sinx=sin((x−α)+α).

Using the identity

sin⁡(u+v)=sin⁡ucos⁡v+cos⁡usin⁡v,\sin(u+v)=\sin u\cos v+\cos u\sin v,sin(u+v)=sinucosv+cosusinv,

we get

sin⁡x=sin⁡(x−α)cos⁡α+cos⁡(x−α)sin⁡α.\sin x = \sin(x-\alpha)\cos\alpha + \cos(x-\alpha)\sin\alpha.sinx=sin(x−α)cosα+cos(x−α)sinα.
  1. Divide by sin⁡(x−α)\sin(x-\alpha)sin(x−α):
sin⁡xsin⁡(x−α)=cos⁡α+sin⁡α cos⁡(x−α)sin⁡(x−α).\frac{\sin x}{\sin(x-\alpha)} = \cos\alpha + \sin\alpha\,\frac{\cos(x-\alpha)}{\sin(x-\alpha)}.sin(x−α)sinx​=cosα+sinαsin(x−α)cos(x−α)​.

So,

sin⁡xsin⁡(x−α)=cos⁡α+sin⁡α cot⁡(x−α).\frac{\sin x}{\sin(x-\alpha)} = \cos\alpha + \sin\alpha\,\cot(x-\alpha).sin(x−α)sinx​=cosα+sinαcot(x−α).
  1. Integrate term by term:
∫sin⁡xsin⁡(x−α) dx=∫cos⁡α dx+sin⁡α∫cot⁡(x−α) dx.\int \frac{\sin x}{\sin(x-\alpha)}\,dx = \int \cos\alpha\,dx + \sin\alpha\int \cot(x-\alpha)\,dx.∫sin(x−α)sinx​dx=∫cosαdx+sinα∫cot(x−α)dx.

Since cos⁡α\cos\alphacosα and sin⁡α\sin\alphasinα are constants,

=xcos⁡α+sin⁡α∫cot⁡(x−α) dx.= x\cos\alpha + \sin\alpha\int \cot(x-\alpha)\,dx.=xcosα+sinα∫cot(x−α)dx.

Now,

∫cot⁡(x−α) dx=log⁡∣sin⁡(x−α)∣.\int \cot(x-\alpha)\,dx = \log|\sin(x-\alpha)|.∫cot(x−α)dx=log∣sin(x−α)∣.

Hence,

∫sin⁡xsin⁡(x−α) dx=xcos⁡α+sin⁡αlog⁡∣sin⁡(x−α)∣+C.\int \frac{\sin x}{\sin(x-\alpha)}\,dx = x\cos\alpha + \sin\alpha\log|\sin(x-\alpha)| + C.∫sin(x−α)sinx​dx=xcosα+sinαlog∣sin(x−α)∣+C.
  1. Comparing with
Ax+Blog⁡sin⁡(x−α)+C,Ax + B\log \sin(x-\alpha) + C,Ax+Blogsin(x−α)+C,

we obtain

A=cos⁡α,B=sin⁡α.A=\cos\alpha, \qquad B=\sin\alpha.A=cosα,B=sinα.
  1. Therefore,
(A,B)=(cos⁡α,sin⁡α).(A,B)=\left(\cos\alpha,\sin\alpha\right).(A,B)=(cosα,sinα).

This matches option B.

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