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Area Under the Curves question

2024 · 6 Apr · Shift 2 · Q39
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  5. /2024 · 6 Apr · Shift 2 · Q39

Area Under the Curves question

2024 · 6 Apr · Shift 2 · Q39

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
If the area of the region {(x,y):ax2≤y≤1x,1≤x≤2,0<a<1}\left\{(x, y): \frac{\mathrm{a}}{x^2} \leq y \leq \frac{1}{x}, 1 \leq x \leq 2,0\lt \mathrm{a}\lt 1\right\}{(x,y):x2a​≤y≤x1​,1≤x≤2,0<a<1} is (log⁡e2)−17\left(\log _{\mathrm{e}} 2\right)-\frac{1}{7}(loge​2)−71​ then the value of 7a−37 \mathrm{a}-37a−3 is equal to :
  1. A
    1
  2. B
    0
  3. C
    2
  4. D
    −-− 1
View written solutionFree

Correct answer: D

  1. Set up the area integral

The region is bounded by ax2≤y≤1x,1≤x≤2,0<a<1.\frac{a}{x^2} \le y \le \frac{1}{x}, \qquad 1 \le x \le 2, \qquad 0<a<1.x2a​≤y≤x1​,1≤x≤2,0<a<1.

So the area is A=∫12(1x−ax2)dx.A=\int_1^2 \left(\frac{1}{x}-\frac{a}{x^2}\right)dx.A=∫12​(x1​−x2a​)dx.

Given that A=ln⁡2−17.A=\ln 2-\frac{1}{7}.A=ln2−71​.


  1. Evaluate the integral

We compute: ∫121x dx=ln⁡2\int_1^2 \frac{1}{x}\,dx=\ln 2∫12​x1​dx=ln2 and ∫121x2 dx=∫12x−2 dx=[−x−1]12=−12−(−1)=12.\int_1^2 \frac{1}{x^2}\,dx=\int_1^2 x^{-2}\,dx=\left[-x^{-1}\right]_1^2= -\frac12-(-1)=\frac12.∫12​x21​dx=∫12​x−2dx=[−x−1]12​=−21​−(−1)=21​.

Therefore, A=ln⁡2−a(12)=ln⁡2−a2.A=\ln 2-a\left(\frac12\right)=\ln 2-\frac{a}{2}.A=ln2−a(21​)=ln2−2a​.


  1. Use the given area

Since ln⁡2−a2=ln⁡2−17,\ln 2-\frac{a}{2}=\ln 2-\frac{1}{7},ln2−2a​=ln2−71​, we get a2=17\frac{a}{2}=\frac{1}{7}2a​=71​ so a=27.a=\frac{2}{7}.a=72​.


  1. Find 7a−37a-37a−3

7a−3=7⋅27−3=2−3=−1.7a-3=7\cdot \frac{2}{7}-3=2-3=-1.7a−3=7⋅72​−3=2−3=−1.


  1. Match with the options

The value is −1,-1,−1, which corresponds to Option D.

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