Q41 / 91 · KVPY 2018 (SB/SX) · Mathematics

A rectangle with its sides parallel to the X-axis and Y-axis is inscribed in the region bounded by the curves y=x2-4 and 2y=4-x2. The maximum possible area of such a rectangle is closest to the integer.

  • A
    10
  • Correct answer
    B
    9
  • C
    8
  • D
    7

Solution

Equation of parabola

y=x2-4 and 2y=4-x2

Figure

Let point B=h,4-h22

A=-h,4-h22

C=h1h2-4

D=-h,h2-4

Area of rectangle

ABCD=AB×BC=2h×4-h22-h2+4

⇒A=12h-3h3⇒dAdh=12-9h2

For maxima or minima

dAdh=0⇒12-9h2=0

h=±23 maximum at h=23

∴A=1223-3233=243-83

=163=1633

A-16×1733=9.22

A=9.22=9

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

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