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In the XY-plane, the area, in sq. units, of the region defined by the inequalities y≥x+4y \geq x+4 and −4≤x2+y2+4(x−y)≤0-4 \leq x^{2}+y^{2}+4(x-y) \leq 0 is

Solution

✅ Correct Option: 2
Solution figure for CAT 2024 QA question 10 (Geometry)

y≥x+4y \geq x+4

−4≤x2+y2+4(x−y)≤0-4 \leq x^{2}+y^{2}+4(x-y) \leq 0


The second condition contains two inequalities:

x2+y2+4(x−y)≥−4x^{2}+y^{2}+4(x-y) \geq -4

x2+y2+4(x−y)≤0x^{2}+y^{2}+4(x-y) \leq 0


Starting with: x2+y2+4(x−y)≥−4x^{2}+y^{2}+4(x-y) \geq -4

Expand: x2+y2+4x−4y≥−4x^{2}+y^{2}+4x-4y \geq -4

Rearrange: x2+y2+4x−4y+4≥0x^{2}+y^{2}+4x-4y+4 \geq 0

Complete the square:

For xx terms: x2+4x=(x+2)2−4x^{2}+4x = (x+2)^{2}-4

For yy terms: y2−4y=(y−2)2−4y^{2}-4y = (y-2)^{2}-4

Substituting: (x+2)2−4+(y−2)2−4+4≥0(x+2)^{2}-4+(y-2)^{2}-4+4 \geq 0

Simplifying: (x+2)2+(y−2)2≥4(x+2)^{2}+(y-2)^{2} \geq 4

This is the region outside or on a circle with center (−2,2)(-2, 2) and radius 22.


Starting with: x2+y2+4(x−y)≤0x^{2}+y^{2}+4(x-y) \leq 0

Expand: x2+y2+4x−4y≤0x^{2}+y^{2}+4x-4y \leq 0

Add 88 to both sides: x2+y2+4x−4y+8≤8x^{2}+y^{2}+4x-4y+8 \leq 8

Complete the square: (x+2)2+(y−2)2≤8(x+2)^{2}+(y-2)^{2} \leq 8

This is the region inside or on a circle with center (−2,2)(-2, 2) and radius 8=22\sqrt{8} = 2\sqrt{2}.


We need the region satisfying all three conditions:

Above the line y=x+4y = x+4

Outside the smaller circle: (x+2)2+(y−2)2≥4(x+2)^{2}+(y-2)^{2} \geq 4

Inside the larger circle: (x+2)2+(y−2)2≤8(x+2)^{2}+(y-2)^{2} \leq 8


Check if the line y=x+4y = x+4 passes through center (−2,2)(-2, 2):

When x=−2x = -2: y=−2+4=2y = -2+4 = 2 ✓

The line passes through the center of both circles, dividing each into two equal halves.


Area of larger circle =π×(8)2=8π= \pi \times (\sqrt{8})^{2} = 8\pi

Area of smaller circle =π×22=4π= \pi \times 2^{2} = 4\pi

Area between circles =8π−4π=4π= 8\pi - 4\pi = 4\pi

Since we only want the half above the line:

Required area =12×4π=2π= \dfrac{1}{2} \times 4\pi = 2\pi

Therefore, the area is 2π2\pi square units.

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