A square with sides of length 6 cm is given. The boundary of the shaded region is defined by two semi-circles whose diameters are the sides of the square, as shown. The area of the shaded region is _______ cm^2. A. 6*pi B. 15 C. 20 D. 9*pi

GATE 2023 · General Aptitude · Circle · medium

Answer: The area of the shaded region is 15 cm^2 (as per the official GATE 2023 answer key, option B).

  1. Area of the square: Area of square = 6^2 = 36 cm^2.
  2. Find the intersection (lens) area of the two semicircles: Centres: C1 = (3,0) and C2 = (6,3). Distance d = sqrt(3^2 + 3^2) = 3*sqrt(2). r = 3. arccos(d/(2r)) = arccos(3*sqrt(2)/6) = arccos(sqrt(2)/2) = pi/4. Lens area = 2*(9)*(pi/4) - (3*sqrt(2)/2)*sqrt(36-18) = 9*pi/2 - (3*sqrt(2)/2)*(3*sqrt(2)) = 9*pi/2 - 9.
  3. Area of union of two semicircles (inclusion-exclusion): Union area = 9*pi/2 + 9*pi/2 - (9*pi/2 - 9) = 9*pi/2 + 9.
  4. Shaded area = square minus union: Shaded = 36 - (9*pi/2 + 9) = 27 - 9*pi/2.