Q22 / 54 · JEE Advanced 2019 (Paper 1) · Mathematics

The area of the region x,y:xy≤8, 1≤y≤x2 is

  • Correct answer
    A
    16loge⁡2-143
  • B
    8loge⁡2-73
  • C
    8loge⁡2-143
  • D
    16loge⁡2-6

Solution

Figure
To draw the inequality, let us draw the equation
xy=8 and y=1 and y=x2
For point of intersection
(i) xy=8 and y=1⇒A(8,1)
(ii) xy=8 and y=x2 ⇒x3=8 ⇒x=2 ⇒B(2, 4)
(iii) y=x2 and y=1⇒C1, 1 and D 1, 1
Now xy≤8⇒ region which contains origin
y≥1⇒ region above line y=1
y≤x2⇒ region outside the parabola
Now required area
Method I:
Using x-axis:
A=∫12x2-1⋅dx+∫288x-1⋅dx
A=x33-x12+8lnx-x28
A=-143+16ln2
Method II:
Using y-axis:
A=∫148y-y⋅dy
A=8lny-23y3214⇒A=-143+16ln2
Note: The question should include bounded area term as in 2nd quadrant there exist a area which satisfy the inequality and is unbounded.

Hence, the correct answer is option: 'A'.

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