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Definite Integration question

2023 · 25 Jan · Shift 2 · Q42
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Definite Integration question

2023 · 25 Jan · Shift 2 · Q42

JEE MainMathematicsDefinite IntegrationNumerical+4 / −1
If ∫133∣log⁡ex∣dx=mnlog⁡e(n2e)\int\limits_{{1 \over 3}}^3 {|{{\log }_e}x|dx = {m \over n}{{\log }_e}\left( {{{{n^2}} \over e}} \right)}31​∫3​∣loge​x∣dx=nm​loge​(en2​), where m and n are coprime natural numbers, then m2+n2−5{m^2} + {n^2} - 5m2+n2−5 is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 20

  1. We need to evaluate I=∫1/33∣ln⁡x∣ dx.I=\int_{1/3}^{3}|\ln x|\,dx.I=∫1/33​∣lnx∣dx.

  2. Since

  • ln⁡x<0\ln x<0lnx<0 for 0<x<10<x<10<x<1,
  • ln⁡x>0\ln x>0lnx>0 for x>1x>1x>1, we split the integral at x=1x=1x=1: I=∫1/31(−ln⁡x) dx+∫13ln⁡x dx.I=\int_{1/3}^{1}(-\ln x)\,dx+\int_{1}^{3}\ln x\,dx.I=∫1/31​(−lnx)dx+∫13​lnxdx.
  1. Use the standard result ∫ln⁡x dx=xln⁡x−x+C.\int \ln x\,dx=x\ln x-x+C.∫lnxdx=xlnx−x+C. Hence, ∫(−ln⁡x) dx=−(xln⁡x−x)=−xln⁡x+x.\int (-\ln x)\,dx=-(x\ln x-x)=-x\ln x+x.∫(−lnx)dx=−(xlnx−x)=−xlnx+x.

  2. First part: I1=∫1/31(−ln⁡x) dx=[−xln⁡x+x]1/31.I_1=\int_{1/3}^{1}(-\ln x)\,dx=[-x\ln x+x]_{1/3}^{1}.I1​=∫1/31​(−lnx)dx=[−xlnx+x]1/31​. Now,

  • at x=1x=1x=1: −1⋅ln⁡1+1=1-1\cdot \ln 1+1=1−1⋅ln1+1=1,
  • at x=1/3x=1/3x=1/3: −13ln⁡(13)+13=13ln⁡3+13.-\frac13\ln\left(\frac13\right)+\frac13=\frac13\ln 3+\frac13.−31​ln(31​)+31​=31​ln3+31​. So, I1=1−(13ln⁡3+13)=23−13ln⁡3.I_1=1-\left(\frac13\ln 3+\frac13\right)=\frac23-\frac13\ln 3.I1​=1−(31​ln3+31​)=32​−31​ln3.
  1. Second part: I2=∫13ln⁡x dx=[xln⁡x−x]13.I_2=\int_1^3 \ln x\,dx=[x\ln x-x]_1^3.I2​=∫13​lnxdx=[xlnx−x]13​. Now,
  • at x=3x=3x=3: 3ln⁡3−33\ln 3-33ln3−3,
  • at x=1x=1x=1: 0−1=−10-1=-10−1=−1. Thus, I2=(3ln⁡3−3)−(−1)=3ln⁡3−2.I_2=(3\ln 3-3)-(-1)=3\ln 3-2.I2​=(3ln3−3)−(−1)=3ln3−2.
  1. Add both parts: I=I1+I2=(23−13ln⁡3)+(3ln⁡3−2).I=I_1+I_2=\left(\frac23-\frac13\ln 3\right)+(3\ln 3-2).I=I1​+I2​=(32​−31​ln3)+(3ln3−2). So, I=-\frac43+\frac83\ln 3= rac83\ln 3-\frac43. Factor 43\frac4334​: I=43(2ln⁡3−1).I=\frac43(2\ln 3-1).I=34​(2ln3−1).

  2. We are given I=mnln⁡(n2e).I=\frac{m}{n}\ln\left(\frac{n^2}{e}\right).I=nm​ln(en2​). Now, ln⁡(n2e)=ln⁡(n2)−1=2ln⁡n−1.\ln\left(\frac{n^2}{e}\right)=\ln(n^2)-1=2\ln n-1.ln(en2​)=ln(n2)−1=2lnn−1. We want mn(2ln⁡n−1)=43(2ln⁡3−1).\frac{m}{n}(2\ln n-1)=\frac43(2\ln 3-1).nm​(2lnn−1)=34​(2ln3−1). This matches directly if n=3,mn=43⇒m=4.n=3,\qquad \frac{m}{n}=\frac43 \Rightarrow m=4.n=3,nm​=34​⇒m=4. Clearly, m=4m=4m=4 and n=3n=3n=3 are coprime.

  3. Therefore, m2+n2−5=42+32−5=16+9−5=20.{m^2+n^2-5}=4^2+3^2-5=16+9-5=20.m2+n2−5=42+32−5=16+9−5=20.

So the required integer is 20.\boxed{20}.20​.

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