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Definite Integration question

2015 · Shift 0 · Q30
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Definite Integration question

2015 · Shift 0 · Q30

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
The integral ∫24log⁡ x2log⁡x2+log⁡(36−12x+x2)dx\int\limits_2^4 {{{\log \,{x^2}} \over {\log {x^2} + \log \left( {36 - 12x + {x^2}} \right)}}dx}2∫4​logx2+log(36−12x+x2)logx2​dx is equal to :
  1. A
    111
  2. B
    666
  3. C
    222
  4. D
    444
View written solutionFree

Correct answer: A

  1. Rewrite the denominator

We need to evaluate

I=∫24log⁡x2log⁡x2+log⁡(36−12x+x2) dx.I=\int_2^4 \frac{\log x^2}{\log x^2+\log(36-12x+x^2)}\,dx.I=∫24​logx2+log(36−12x+x2)logx2​dx.

Notice that

36−12x+x2=x2−12x+36=(x−6)2=(6−x)2.36-12x+x^2=x^2-12x+36=(x-6)^2=(6-x)^2.36−12x+x2=x2−12x+36=(x−6)2=(6−x)2.

So

I=∫24log⁡x2log⁡x2+log⁡(6−x)2 dx.I=\int_2^4 \frac{\log x^2}{\log x^2+\log(6-x)^2}\,dx.I=∫24​logx2+log(6−x)2logx2​dx.

Using log⁡a+log⁡b=log⁡(ab)\log a+\log b=\log(ab)loga+logb=log(ab),

I=∫24log⁡x2log⁡(x2(6−x)2) dx.I=\int_2^4 \frac{\log x^2}{\log\big(x^2(6-x)^2\big)}\,dx.I=∫24​log(x2(6−x)2)logx2​dx.
  1. Use symmetry with the substitution x↦6−xx\mapsto 6-xx↦6−x

Let

f(x)=log⁡x2log⁡x2+log⁡(6−x)2.f(x)=\frac{\log x^2}{\log x^2+\log(6-x)^2}.f(x)=logx2+log(6−x)2logx2​.

Then

I=∫24f(x) dx.I=\int_2^4 f(x)\,dx.I=∫24​f(x)dx.

Now apply the substitution

x=6−t⇒dx=−dt.x=6-t \quad \Rightarrow \quad dx=-dt.x=6−t⇒dx=−dt.

When x=2x=2x=2, t=4t=4t=4; when x=4x=4x=4, t=2t=2t=2. Hence

I=∫42f(6−t)(−dt)=∫24f(6−t) dt.I=\int_4^2 f(6-t)(-dt)=\int_2^4 f(6-t)\,dt.I=∫42​f(6−t)(−dt)=∫24​f(6−t)dt.

Renaming ttt as xxx,

I=∫24f(6−x) dx.I=\int_2^4 f(6-x)\,dx.I=∫24​f(6−x)dx.

Now compute

f(6−x)=log⁡(6−x)2log⁡(6−x)2+log⁡x2.f(6-x)=\frac{\log(6-x)^2}{\log(6-x)^2+\log x^2}.f(6−x)=log(6−x)2+logx2log(6−x)2​.

Therefore,

f(x)+f(6−x)=log⁡x2log⁡x2+log⁡(6−x)2+log⁡(6−x)2log⁡x2+log⁡(6−x)2=1.f(x)+f(6-x)=\frac{\log x^2}{\log x^2+\log(6-x)^2}+\frac{\log(6-x)^2}{\log x^2+\log(6-x)^2}=1.f(x)+f(6−x)=logx2+log(6−x)2logx2​+logx2+log(6−x)2log(6−x)2​=1.
  1. Add the two expressions for III

Since

I=∫24f(x) dxI=\int_2^4 f(x)\,dxI=∫24​f(x)dx

and

I=∫24f(6−x) dx,I=\int_2^4 f(6-x)\,dx,I=∫24​f(6−x)dx,

adding,

2I=∫24(f(x)+f(6−x)) dx=∫241 dx=4−2=2.2I=\int_2^4 \big(f(x)+f(6-x)\big)\,dx=\int_2^4 1\,dx=4-2=2.2I=∫24​(f(x)+f(6−x))dx=∫24​1dx=4−2=2.

So

I=1.I=1.I=1.
  1. Check options

Thus the value of the integral is

1.\boxed{1}.1​.

So the correct option is A.

  1. Comparison with stored answer

Stored correct answer: A

Our derived answer: A

Hence, they agree.

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