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Indefinite Integrals question

2025 · 28 Jan · Shift 2 · Q41
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Indefinite Integrals question

2025 · 28 Jan · Shift 2 · Q41

JEE MainMathematicsIndefinite IntegralsMCQ+4 / −1
If f(x)=∫1x1/4(1+x1/4)dx,f(0)=−6f(x)=\int \frac{1}{x^{1 / 4}\left(1+x^{1 / 4}\right)} \mathrm{d} x, f(0)=-6f(x)=∫x1/4(1+x1/4)1​dx,f(0)=−6, then f(1)f(1)f(1) is equal to :
  1. A
    4(log⁡e2−2)4\left(\log _{\mathrm{e}} 2-2\right)4(loge​2−2)
  2. B
    log⁡e22+2\log _{e^2} 2+2loge2​2+2
  3. C
    2−log⁡e22-\log \mathrm{e}^22−loge2
  4. D
    4(log⁡e2+2)4\left(\log _e 2+2\right)4(loge​2+2)
View written solutionFree

Correct answer: A

  1. We need to evaluate f(x)=∫1x1/4(1+x1/4) dxf(x)=\int \frac{1}{x^{1/4}(1+x^{1/4})}\,dxf(x)=∫x1/4(1+x1/4)1​dx with the condition f(0)=−6.f(0)=-6.f(0)=−6. Then we must find f(1)f(1)f(1).

  2. Let t=x1/4  ⟹  x=t4,dx=4t3 dt.t=x^{1/4} \implies x=t^4, \quad dx=4t^3\,dt.t=x1/4⟹x=t4,dx=4t3dt. Also, x1/4=t.x^{1/4}=t.x1/4=t. So the integrand becomes 1t(1+t)⋅4t3 dt=4t21+t dt.\frac{1}{t(1+t)}\cdot 4t^3\,dt=\frac{4t^2}{1+t}\,dt.t(1+t)1​⋅4t3dt=1+t4t2​dt. Hence, f(x)=∫4t21+t dt.f(x)=\int \frac{4t^2}{1+t}\,dt.f(x)=∫1+t4t2​dt.

  3. Divide t2t^2t2 by (1+t)(1+t)(1+t): t2t+1=t−1+1t+1.\frac{t^2}{t+1}=t-1+\frac{1}{t+1}.t+1t2​=t−1+t+11​. Therefore, 4t21+t=4(t−1+1t+1).\frac{4t^2}{1+t}=4\left(t-1+\frac{1}{t+1}\right).1+t4t2​=4(t−1+t+11​). So, f(x)=4∫(t−1+1t+1)dt.f(x)=4\int \left(t-1+\frac{1}{t+1}\right)dt.f(x)=4∫(t−1+t+11​)dt.

  4. Integrate termwise: f(x)=4(t22−t+ln⁡(1+t))+C.f(x)=4\left(\frac{t^2}{2}-t+\ln(1+t)\right)+C.f(x)=4(2t2​−t+ln(1+t))+C. Substituting back t=x1/4t=x^{1/4}t=x1/4, f(x)=2x1/2−4x1/4+4ln⁡(1+x1/4)+C.f(x)=2x^{1/2}-4x^{1/4}+4\ln(1+x^{1/4})+C.f(x)=2x1/2−4x1/4+4ln(1+x1/4)+C.

  5. Use the condition f(0)=−6f(0)=-6f(0)=−6. At x=0x=0x=0, x1/2=0,x1/4=0,ln⁡(1+0)=0.x^{1/2}=0, \quad x^{1/4}=0, \quad \ln(1+0)=0.x1/2=0,x1/4=0,ln(1+0)=0. Thus, f(0)=C=−6.f(0)=C=-6.f(0)=C=−6. So, f(x)=2x1/2−4x1/4+4ln⁡(1+x1/4)−6.f(x)=2x^{1/2}-4x^{1/4}+4\ln(1+x^{1/4})-6.f(x)=2x1/2−4x1/4+4ln(1+x1/4)−6.

  6. Now compute f(1)f(1)f(1): f(1)=2(1)−4(1)+4ln⁡(1+1)−6.f(1)=2(1)-4(1)+4\ln(1+1)-6.f(1)=2(1)−4(1)+4ln(1+1)−6. That is, f(1)=2−4+4ln⁡2−6=4ln⁡2−8.f(1)=2-4+4\ln 2-6=4\ln 2-8.f(1)=2−4+4ln2−6=4ln2−8. Factorizing, f(1)=4(ln⁡2−2).f(1)=4(\ln 2-2).f(1)=4(ln2−2).

  7. Compare with the options: Option A is 4(log⁡e2−2),4(\log_e 2-2),4(loge​2−2), which matches exactly.

Therefore, the correct answer is A.

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