Q.To reduce traffic and avoid red lights, roundabouts are often built on busy roads. Such a roundabout is built such that its boundary C1:x2+y2=64 is given. In the middle of this roundabout, there is a circular pond with a fountain, whose equation is C2:x2+y2=4. Based on the above information, answer the following questions:
(A) Find the area enclosed by the entire roundabout using integration. 2
(B) Find the area enclosed by the circular pond using integration. 2
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Area Under Curve
Area Under a Curve
How do you measure the area of a region whose top edge is curved rather than a straight line? For rectangles and triangles we have formulas, but a shape bounded above by y=f(x) has no simple side lengths to plug in. The definite integral is the tool built exactly for this.
The core idea: area as a limit of strips
Take the region under y=f(x) (with f(x)≥0), above the x-axis, between x=a and x=b. Slice it into many thin vertical strips. A strip at position x with tiny width dx is almost a rectangle of height f(x), so its area is about f(x)dx. Add up all the strips and let their width shrink to zero: the sum becomes the definite integral
Area=∫abf(x)dx.
This is why the integral is the area — it is the exact total of infinitely many infinitesimally thin rectangles.
How to compute it
Find an antiderivative F(x) (so F′(x)=f(x)) and evaluate at the two limits — the Fundamental Theorem of Calculus:
∫abf(x)dx=F(b)−F(a).
For example, the area under y=x2 from 0 to 1 is [3x3]01=31.
Cases you must handle carefully
The integral gives signed area. Where the curve dips below the x-axis, f(x)<0 and ∫fdx comes out negative. For the geometric (positive) area of such a stretch, integrate the absolute value or take the magnitude of that piece: Area=∫ab∣f(x)∣dx.
If a curve crosses the x-axis inside [a,b], split the integral at each crossing and add the sizes of the parts.
Area with respect to the y-axis
When the region is bounded by a curve x=g(y) and the y-axis between y=c and y=d, slice horizontally instead: …
Part (b)Concept understanding — Area Under Curve
Area Under a Curve
How do you measure the area of a region whose top edge is curved rather than a straight line? For rectangles and triangles we have formulas, but a shape bounded above by y=f(x) has no simple side lengths to plug in. The definite integral is the tool built exactly for this.
The core idea: area as a limit of strips
Take the region under y=f(x) (with f(x)≥0), above the x-axis, between x=a and x=b. Slice it into many thin vertical strips. A strip at position x with tiny width dx is almost a rectangle of height f(x), so its area is about f(x)dx. Add up all the strips and let their width shrink to zero: the sum becomes the definite integral
Area=∫abf(x)dx.
This is why the integral is the area — it is the exact total of infinitely many infinitesimally thin rectangles.
How to compute it
Find an antiderivative F(x) (so F′(x)=f(x)) and evaluate at the two limits — the Fundamental Theorem of Calculus:
∫abf(x)dx=F(b)−F(a).
For example, the area under y=x2 from 0 to 1 is [3x3]01=31.
Cases you must handle carefully
The integral gives signed area. Where the curve dips below the x-axis, f(x)<0 and ∫fdx comes out negative. For the geometric (positive) area of such a stretch, integrate the absolute value or take the magnitude of that piece: Area=∫ab∣f(x)∣dx.
If a curve crosses the x-axis inside [a,b], split the integral at each crossing and add the sizes of the parts.
Area with respect to the y-axis
When the region is bounded by a curve x=g(y) and the y-axis between y=c and y=d, slice horizontally instead: …
Part (a)
(i) C1:x2+y2=64 and C2:x2+y2=4 are concentric circles (centre O) of radii 8 and 2; C2 lies inside C1.
(ii) C1:y=±64−x2; C2:y=±4−x2.
(iii)(A) Area of the roundabout (C1): …
C1,C2 are concentric circles of radii 8 and 2, with y=±64−x2 and y=±4−x2.
Part (a): area of the roundabout (C1) =64π square units.
Part (b): area of the pond (C2) =4π square units.
For a circle x2+y2=r2, integrating the upper half y=r2−x2 over [−r,r] and doubling recovers the area πr2; equivalently, four times the first-quadrant integral.
Part (a)
(i) C1:x2+y2=64 is a circle centred at the origin with radius 8 (the roundabout boundary); C2:x2+y2=4 is concentric with radius 2 (the pond). Draw two concentric circles, the smaller inside the larger.
(ii) Solving for y: C1:y=±64−x2 and C2:y=±4−x2.
(iii)(A) Using symmetry (four equal quadrants) and ∫a2−x2dx=2xa2−x2+2a2sin−1ax with a=8: …
Method: Area of a circle (and a ring between two circles) by integration
Use this whenever a region is enclosed by a circle x2+y2=r2, or is the ring left between an outer and an inner concentric circle.
Steps
Step 1: Read each radius.
From x2+y2=r2 the radius is r=r2. If two circles are given, name the larger radius R and the smaller r.
Step 2: Set up one circle's area with symmetry.
A circle is symmetric about both axes, so integrate the first-quadrant arc y=r2−x2 and multiply by 4:
A=4∫0rr2−x2dx,
using
∫r2−x2dx=2xr2−x2+2r2sin−1rx+C. …
Common Mistakes
Mistake 1: Misreading the radius from x2+y2=64
Students take the radius as 64 instead of 8. Why it's wrong: the standard form is x2+y2=r2, so r2=64 gives r=8 (and r2=4 gives r=2). Correct approach: always square-root the constant on the right.
Mistake 2: Subtracting the pond when the question asks for the whole roundabout
Part (iii)(A) asks for the area enclosed by the entire roundabout C1, which is the full disc 64π — not the ring between the two circles. Why it's wrong: subtracting the pond (64π−4π=60π) answers a different question. Correct approach: read carefully — the full roundabout is 64π, the pond alone is 4π. …
Showing the 12 most recent of 16 on this concept.
- KEAM 2026Set eng-2026-04174 marksMCQQ.A curve with equation y=x3−8x2+16x meets the x-axis at the origin O and at a point A. Then the area of the region, bounded by the curve and the straight-line segment OA, is (A) 361 (B) 362 (C) 364 (D) 365 (E) 368
›Reveal solutionSolution
Factor to find the roots, then integrate from 0 to 4.
y=x3−8x2+16x=x(x−4)2, so the curve meets the x-axis at O(0,0) and A(4,0), and y≥0 on [0,4]. The area between the curve and OA (the x-axis) is …
- KEAM 2026Set eng-2026-04184 marksMCQQ.The area of the region bounded by y=x, y=−x, and x=4 (in square units) is (A) 38 (B) 320 (C) 340 (D) 326 (E) 332
›Reveal solutionSolution
Between x=0 and x=4 the upper curve is y=x and the lower is y=−x; integrating their difference gives 340.
For 0≤x≤4, y=x lies above y=−x, so
A=∫04(x−(−x))dx=∫04(x+x)dx=[32x3/2+2x2]04. …
- KEAM 2026Set eng-2026-04214 marksMCQQ.The area of the region bounded by the lines, y=x+2, x=0, x=1 and y=0 is (A) 2 sq.units (B) 25 sq.units (C) 29 sq.units (D) 9 sq.units (E) 12 sq.units
›Reveal solutionSolution
The region is under y=x+2 from x=0 to 1; its area is ∫01(x+2)dx=25.
On [0,1] the line y=x+2 lies above y=0. The bounded area is
∫01(x+2)dx=[2x2+2x]01=21+2=25 sq. units. …
- KEAM 2025Set eng-2025-04234 marksMCQQ.The area of the region bounded by y=x5/2 and y=x (in square units) is (A) 73 (B) 72 (C) 143 (D) 145 (E) 74
›Reveal solutionSolution
The curves meet at x=0 and x=1, and the enclosed area is 1/2 - 2/7 = 3/14.
Concept and Intuition
Between the intersection points, the line y=x lies above y=x^{5/2} on (0,1), so the area is the integral of the difference.
Step-by-Step Solution
- Intersection: x^{5/2} = x gives x(x^{3/2}-1)=0, so x = 0 and x = 1.
- On (0,1), x > x^{5/2}, so area = integral from 0 to 1 of (x - x^{5/2}) dx. …
- KEAM 2025Set eng-2025-04264 marksMCQQ.The area bounded by y=x−1,1≤x≤2,y=0 (in sq.units) is (A) 2 (B) 1 (C) 21 (D) 4 (E) 41
›Reveal solutionSolution
Integrate x−1 from 1 to 2.
On [1,2] the line y=x−1 is nonnegative, so the area between it and y=0 is …
- KEAM 2025Set eng-2025-04284 marksMCQQ.Area of the region bounded by the function f(x)={x−x+6x≤3x>3 with the x-axis (in square units) in the first quadrant is (A) 18 (B) 9 (C) 6 (D) 3 (E) 4.5
›Reveal solutionSolution
The graph forms a triangle with base 6 (from 0 to 6) and peak height 3 at x=3; area =21(6)(3)=9.
For x≤3, y=x rises from (0,0) to (3,3). For x>3, y=−x+6 falls from (3,3) to (6,0). Together with the x-axis this bounds a triangle with base 6 (from x=0 to x=6) and height 3. Computing directly: …
- KEAM 2025Set eng-2025-04294 marksMCQQ.Area of the region bounded by y=∣x∣ and x=4 is (A) 4 sq.units (B) 6 sq.units (C) 8 sq.units (D) 12 sq.units (E) 13 sq.units
›Reveal solutionSolution
The region under y=∣x∣ up to x=4 (first-quadrant triangle) has area 21×4×4=8 sq. units.
Setting up the region. The curve y=∣x∣ for x≥0 is the straight line y=x. Bounded by the x-axis and the vertical line x=4, this traces a right triangle with vertices (0,0), (4,0) and (4,4).
Computing the area. …
- KEAM 2024Set eng-2024-06054 marksMCQQ.The area bounded by the curves y=x2 and y=2x in the first quadrant, is equal to (A) 32 (B) 34 (C) 31 (D) 38 (E) 37
›Reveal solutionSolution
Intersection at x=0,2; integrate the difference of the upper and lower curves.
Set x2=2x⇒x=0 or x=2. On (0,2) the line y=2x lies above y=x2. So …
- KEAM 2024Set eng-2024-06064 marksMCQQ.The area bounded by the parabola y=x2+2 and the lines y=x, x=1 and x=2 is (in square units) (A) 631 (B) 629 (C) 625 (D) 617 (E) 613
›Reveal solutionSolution
Integrate (upper curve − lower line) over [1,2].
On [1,2], x2+2>x, so
A=∫12(x2+2−x)dx=[3x3+2x−2x2]12. …
- KEAM 2024Set eng-2024-06074 marksMCQQ.The area bounded by the parabola y=x2+4 and the straight line passing through the points (−1,2) and (1,6) is (in square units) (A) 320 (B) 34 (C) 38 (D) 316 (E) 314
›Reveal solutionSolution
Find the line, its intersections with the parabola, and integrate the gap.
Slope of the line =1−(−1)6−2=2, so y−6=2(x−1)⇒y=2x+4.
Intersections with y=x2+4:
x2+4=2x+4⇒x2−2x=0⇒x=0,2.
On (0,2) the line lies above the parabola (at x=1: line 6, parabola 5): …
- KEAM 2024Set eng-2024-06094 marksMCQQ.The area bounded by the curves y=2x and y=x2 (in square units) is (A) 32 (B) 31 (C) 34 (D) 23 (E) 0
›Reveal solutionSolution
The line and parabola meet at x=0,2; between them the line is above, so the area is ∫02(2x−x2)dx=34.
Setting 2x=x2 gives x=0 and x=2. On (0,2), 2x>x2, so …
- KEAM 2024Set eng-2024-06094 marksMCQQ.The area of the smaller segment cut-off from the circle x2+y2=25 by x=3 is (in Sq. units) (A) 75cos−1(53)−12 (B) 25cos−1(53)−24 (C) 25cos−1(53)−12 (D) 25cos−1(53)−6 (E) 50cos−1(53)−12
›Reveal solutionSolution
Integrating 225−x2 from x=3 to 5 and converting sin−1 to cos−1 gives 25cos−153−12.
The smaller segment lies to the right of x=3:
A=∫35225−x2dx=2[2x25−x2+225sin−15x]35.
At x=5: 0+225⋅2π; at x=3: 23⋅4+225sin−153=6+225sin−153. …
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