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Indefinite Integrals question

2021 · 18 Mar · Shift 1 · Q28
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  5. /2021 · 18 Mar · Shift 1 · Q28

Indefinite Integrals question

2021 · 18 Mar · Shift 1 · Q28

JEE MainMathematicsIndefinite IntegralsMCQ+4 / −1
The integral ∫(2x−1)cos⁡(2x−1)2+54x2−4x+6dx\int {{{(2x - 1)\cos \sqrt {{{(2x - 1)}^2} + 5} } \over {\sqrt {4{x^2} - 4x + 6} }}} dx∫4x2−4x+6​(2x−1)cos(2x−1)2+5​​dx is equal to (where c is a constant of integration)
  1. A
    12sin⁡(2x−1)2+5+c{1 \over 2}\sin \sqrt {{{(2x - 1)}^2} + 5} + c21​sin(2x−1)2+5​+c
  2. B
    12cos⁡(2x+1)2+5+c{1 \over 2}\cos \sqrt {{{(2x + 1)}^2} + 5} + c21​cos(2x+1)2+5​+c
  3. C
    12cos⁡(2x−1)2+5+c{1 \over 2}\cos \sqrt {{{(2x - 1)}^2} + 5} + c21​cos(2x−1)2+5​+c
  4. D
    12sin⁡(2x+1)2+5+c{1 \over 2}\sin \sqrt {{{(2x + 1)}^2} + 5} + c21​sin(2x+1)2+5​+c
View written solutionFree

Correct answer: A

  1. Rewrite the denominator

Given I=∫(2x−1)cos⁡(2x−1)2+54x2−4x+6 dxI=\int \frac{(2x-1)\cos\sqrt{(2x-1)^2+5}}{\sqrt{4x^2-4x+6}}\,dxI=∫4x2−4x+6​(2x−1)cos(2x−1)2+5​​dx

Notice that 4x2−4x+6=(2x−1)2+54x^2-4x+6=(2x-1)^2+54x2−4x+6=(2x−1)2+5

because (2x−1)2+5=4x2−4x+1+5=4x2−4x+6.(2x-1)^2+5=4x^2-4x+1+5=4x^2-4x+6.(2x−1)2+5=4x2−4x+1+5=4x2−4x+6.

So the integral becomes I=∫(2x−1)cos⁡(2x−1)2+5(2x−1)2+5 dx.I=\int \frac{(2x-1)\cos\sqrt{(2x-1)^2+5}}{\sqrt{(2x-1)^2+5}}\,dx.I=∫(2x−1)2+5​(2x−1)cos(2x−1)2+5​​dx.

  1. Use substitution

Let t=(2x−1)2+5.t=\sqrt{(2x-1)^2+5}.t=(2x−1)2+5​. Then t2=(2x−1)2+5.t^2=(2x-1)^2+5.t2=(2x−1)2+5.

Differentiate both sides: 2t dtdx=2(2x−1)⋅2=4(2x−1).2t\,\frac{dt}{dx}=2(2x-1)\cdot 2=4(2x-1).2tdxdt​=2(2x−1)⋅2=4(2x−1).

Hence t dt=2(2x−1) dxt\,dt=2(2x-1)\,dxtdt=2(2x−1)dx so (2x−1)t dx=12 dt.\frac{(2x-1)}{t}\,dx=\frac{1}{2}\,dt.t(2x−1)​dx=21​dt. Since t=(2x−1)2+5t=\sqrt{(2x-1)^2+5}t=(2x−1)2+5​, the integral becomes I=∫cos⁡t((2x−1)tdx)=∫cos⁡t⋅12 dt.I=\int \cos t\left(\frac{(2x-1)}{t}dx\right)=\int \cos t\cdot \frac12\,dt.I=∫cost(t(2x−1)​dx)=∫cost⋅21​dt.

Thus I=12∫cos⁡t dt=12sin⁡t+c.I=\frac12\int \cos t\,dt=\frac12\sin t+c.I=21​∫costdt=21​sint+c.

  1. Substitute back

I=12sin⁡(2x−1)2+5+c.I=\frac12\sin\sqrt{(2x-1)^2+5}+c.I=21​sin(2x−1)2+5​+c.

  1. Match with the options

This is exactly Option A: 12sin⁡(2x−1)2+5+c\boxed{\frac12\sin\sqrt{(2x-1)^2+5}+c}21​sin(2x−1)2+5​+c​

  1. Verification by differentiation

Differentiate F(x)=12sin⁡(2x−1)2+5.F(x)=\frac12\sin\sqrt{(2x-1)^2+5}.F(x)=21​sin(2x−1)2+5​.

Then F′(x)=12cos⁡(2x−1)2+5⋅12(2x−1)2+5⋅4(2x−1)F'(x)=\frac12\cos\sqrt{(2x-1)^2+5}\cdot \frac{1}{2\sqrt{(2x-1)^2+5}}\cdot 4(2x-1)F′(x)=21​cos(2x−1)2+5​⋅2(2x−1)2+5​1​⋅4(2x−1) =(2x−1)cos⁡(2x−1)2+5(2x−1)2+5=\frac{(2x-1)\cos\sqrt{(2x-1)^2+5}}{\sqrt{(2x-1)^2+5}}=(2x−1)2+5​(2x−1)cos(2x−1)2+5​​ =(2x−1)cos⁡(2x−1)2+54x2−4x+6,=\frac{(2x-1)\cos\sqrt{(2x-1)^2+5}}{\sqrt{4x^2-4x+6}},=4x2−4x+6​(2x−1)cos(2x−1)2+5​​, which matches the integrand.

Therefore, the answer is correct.

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