Q.Find the area of the region bounded by the line x=2 and the parabola y2=8x.
Concept understanding — Area Under Parabola
Area Under a Parabola
Picture the simplest parabola, y=x2: a smooth U opening upward with its lowest point at the origin. Suppose we want the area trapped between this curve, the x-axis, and the vertical lines x=0 and x=1 — the area under the parabola on [0,1].
A rough estimate helps. A rectangle of base 1 and height 1 gives area 1 — too big, since the curve sits well below its top. A triangle gives 21×1×1=0.5 — too small, since the curve bulges above the straight edge. So the true area lies somewhere between 0.5 and 1.
Calculus pins it down exactly. The area under a curve y=f(x) from x=a to x=b (with f(x)≥0) is the definite integral
Area=∫abf(x)dx.
For y=x2 from 0 to 1 we use the power rule
∫xndx=n+1xn+1+C(n=−1)
so, with n=2,
∫01x2dx=[3x3]01=31−0=31.
The exact area is 31 square units (about 0.333) — comfortably between our two guesses.
The area is exactly one-third of the bounding rectangle. In general, under y=x2 from 0 to a the area is 3a3, i.e. one-third of the a×a2 rectangle.
The general statement
For y=kx2 (k constant), the area from x=a to x=b is
∫abkx2dx=k⋅3b3−a3.
If the parabola is shifted, such as y=x2+c, integrate term by term. If it opens sideways, such as x=y2, integrate with respect to y instead.
The cube formula 3b3−a3 applies only to y=x2 (or a constant multiple). For a full quadratic y=ax2+bx+c you must integrate the whole expression — never apply the cube formula to the x2 term alone.
The key takeaway: integration converts a curved boundary into an exact number, and for the basic parabola y=x2 from 0 to a that number is simply 3a3.
Finding the area under a parabola using definite integration is a foundational example in the CBSE Class 12 Application of Integrals chapter, and "area under curve y = x^2 using integration" is a commonly searched topic for board exam revision. This same integration approach scales up to the more general area-bounded-by-curves questions tested in JEE Main.
Concept: Area Under a Parabola — we integrate the horizontal strips between the curve and the line.
The parabola y2=8x opens to the right. At x=2, the y-coordinates are y=±8⋅2=±4.
The region is symmetric about the x-axis, so we find the area in the upper half and double it.
Step 1: Express x in terms of y:
x=8y2.
Step 2: For a fixed y, the horizontal strip runs from the parabola x=y2/8 to the line x=2. Strip length = 2−8y2.
Step 3: Integrate from y=−4 to y=4, using symmetry:
Area=2∫04(2−8y2)dy=2[2y−24y3]04=2(8−2464)=2(8−38)=2⋅316=332.
The area is 332 square units.
Integrating the horizontal strips of width (2−8y2) from y=−4 to y=4 gives an area of 332 square units.
Concept
The parabola y2=8x opens to the right with vertex at the origin; x=8y2. The vertical line x=2 closes off a region symmetric about the x-axis. Integrating with respect to y (strip width = right boundary − left boundary) is cleanest.
Solution
1. Intersection points. Set 8y2=2⇒y2=16⇒y=±4. So y runs from −4 to 4.
2. Strip width. For a fixed y, the region runs from the parabola x=8y2 to the line x=2, width 2−8y2.
3. Set up and use symmetry (integrand is even):
A=∫−44(2−8y2)dy=2∫04(2−8y2)dy.
4. Evaluate.
2∫04(2−8y2)dy=2[2y−24y3]04=2(8−2464)=2(8−38)=2⋅316=332.
5. Check (integrating in x). A=2∫028xdx=28⋅32x3/202=348(22)=332. Both methods agree.
The area bounded by x=2 and y2=8x is 332 square units.
Method: Area by horizontal strips (integrating with respect to y)
When a sideways parabola y2=4ax is closed off by a vertical line x=c, integrating in y is cleaner than in x because each horizontal strip has two clean x-boundaries.
Steps
Step 1: Express x as a function of y.
From y2=4ax write x=4ay2 — the left boundary of a horizontal strip. The vertical line x=c is the right boundary.
Step 2: Find the y-limits.
Set 4ay2=c to get y=±4ac: the strip heights range symmetrically about the x-axis.
Step 3: Use symmetry and integrate the strip width.
The region is symmetric about the x-axis, so
A=∫−y0y0(c−4ay2)dy=2∫0y0(c−4ay2)dy,
using ∫y2dy=3y3. Substitute the limit y0 to finish. (You can cross-check by integrating 24ax in x from 0 to c.)
Common Mistakes
Mistake 1: Forgetting the region is symmetric and dropping the factor of 2
The line x=2 cuts the parabola at y=+4 and y=−4, so the region lies both above and below the x-axis. Why it's wrong: integrating only the upper half gives 316, half the true answer. Correct approach: double the upper-half area, or integrate y from −4 to 4, giving 332.
Mistake 2: Using the wrong strip length
With horizontal strips, the width is (right boundary − left boundary) =2−8y2. Why it's wrong: writing 8y2−2 makes the length negative and the area wrong. Correct approach: the line x=2 is to the right of the parabola x=8y2, so subtract parabola from line.
Mistake 3: Wrong limits at x=2
At x=2, y2=8(2)=16 so y=±4. Why it's wrong: students take y=±8 or forget to substitute x=2. Correct approach: plug x=2 into y2=8x to get y=±4.
Showing the 12 most recent of 14 on this concept.
- GUJCET 2022Set 081 markMCQQ.The area of the region bounded by the curve y2=4x and the line x=3 is ______. (A) 33 (B) 38 (C) 8 (D) 83
›Reveal solutionSolution
Integrate the full width 24x from 0 to 3.
Concept. For y2=4x, y=±2x, so width =4x.
A=∫034xdx=4⋅32x3/203=38⋅33/2=38⋅33=83.
✓Final answer(D) 83
ANSWER: (D)
- GUJCET 2020Set 071 markMCQQ.The area of the parabola y2=4ax bounded by its latus rectum is ________. (A) 316a2 (B) 34a2 (C) 38a2 (D) 4a2
›Reveal solutionSolution
Integrate y=2ax from x=0 to x=a and double it (symmetry about the axis).
Concept: The latus rectum is the line x=a. Area between the parabola and this chord:
A=2∫0a4axdx=2∫0a2axdx=4a[32x3/2]0a
=4a⋅32a3/2=38a2
✓Final answer(C) 38a2
ANSWER: (C)
- GUJCET 2023Set 091 markMCQQ.Area of the region enclosed by the parabola y=x2 and the line y=x+2 is : (A) 29 (B) 211 (C) 25 (D) 27
›Reveal solutionSolution
Integrate (line − parabola) between their intersection points.
Concept. Intersections of y=x2 and y=x+2: x2−x−2=0⇒x=−1,2.
Solution.
∫−12[(x+2)−x2]dx=[2x2+2x−3x3]−12=310−(−67)=620+7=627=29.
✓Final answer(A) 29
ANSWER: (A)
- GUJCET 2022Set 081 markMCQQ.The area of the region bounded by the two parabolas y=x2 and y2=x is ______. (A) 43 (B) 3 (C) 21 (D) 31
›Reveal solutionSolution
The curves meet at (0,0) and (1,1); integrate x−x2.
Concept. Between x=0 and x=1, y2=x (upper, y=x) lies above y=x2.
A=∫01(x−x2)dx=[32x3/2−3x3]01=32−31=31.
✓Final answer(D) 31
ANSWER: (D)
- GUJCET 2022Set 081 markMCQQ.The area of the parabola x2=12y bounded by its latus rectum is ______. (A) 3 (B) 324 (C) 24 (D) 38
›Reveal solutionSolution
Here 4a=12⇒a=3; the latus rectum is y=3, and the enclosed area is 24.
Concept. x2=12y has a=3; latus rectum at y=3, where x=±6. The region between the parabola and the line:
A=∫−66(3−12x2)dx=2[3x−36x3]06=2(18−6)=24.
✓Final answer(C) 24
ANSWER: (C)
- GUJCET 2021Set 151 markMCQQ.Area of the region bounded by the curve y2=x and the lines x=1, x=4 and X-axis in the first quadrant is (A) 314 (B) 37 (C) 328 (D) 14
›Reveal solutionSolution
Integrate y=x from x=1 to x=4.
Concept — area under a curve. In the first quadrant y2=x gives y=x. The region is bounded by x=1, x=4 and the X-axis:
A=∫14xdx=32[x3/2]14=32(8−1)=314.
✓Final answer(A) 314
ANSWER: (A)
- GUJCET 2021Set 151 markMCQQ.Area of the region bounded by the curve y2=4x, Y-axis and the line y=3 is (A) 2 (B) 39 (C) 49 (D) 29
›Reveal solutionSolution
Integrate along the y-axis: x=y2/4 from y=0 to y=3.
Concept — area bounded by a parabola and the Y-axis. From y2=4x, x=4y2. The region is bounded by the Y-axis and the line y=3:
A=∫03xdy=∫034y2dy=41⋅3y303=1227=49.
✓Final answer(C) 49
ANSWER: (C)
- GUJCET 2024Set 131 markMCQQ.Area of the region bounded by the curve x2=4y, X-axis and the line x=3 is __________. (A) 29 (B) 49 (C) 39 (D) 2
›Reveal solutionSolution
Area =∫03ydx with y=x2/4.
Concept. From x2=4y, y=4x2. Area between the curve, the X-axis and x=3:
A=∫034x2dx=41[3x3]03=41⋅327=49.
✓Final answerOption (B) 49
ANSWER: (B)
- GUJCET 2025Set 031 markMCQQ.Area of the region bounded by the curve x2=4y and the line y=3 is (A) 43 (B) 23 (C) 3 (D) 33
›Reveal solutionSolution
[!TLDR] R2+(22R)2=9R2, (9R2)3/2=27R3 ⇒ ratio =27:1.
Concept
Magnetic field at the centre of a ring is Bc=2Rμ0I and on its axis at distance x is Ba=2(R2+x2)3/2μ0IR2 (NCERT Moving Charges and Magnetism).
Solution
With x=22R:
R2+x2=R2+8R2=9R2,(9R2)3/2=27R3.
Ba=2(27R3)μ0IR2=54Rμ0I.
BaBc=μ0I/(54R)μ0I/(2R)=254=27.
So the ratio is 27:1.
[!ANSWER] (A)
NoteThis solution was worked out by our team and independently cross-checked by a second solve. The official answer key on record for this question could not be confirmed, so please cross-verify with the official paper where possible.
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.The area of the region bounded by the curve y=x2 and the line y=16 is:(a) 128/3(b) 256/3(c) 64/3(d) 512/3
›Reveal solutionSolution
The parabola meets the line y=16 at x=±4; integrate (16−x2) between these limits.
x2=16⇒x=±4. By symmetry,
Area =2∫04(16−x2)dx=2[16x−3x3]04=2(64−364)=2⋅3128=3256.
✓Final answerThe correct option is (b) 256/3.
- GSEB Higher Secondary Certificate (HSC) Examination 2024Set ANNUAL1 markMCQQ.Area of the region bounded by the curve y2=4x, y-axis and the line y=3 is ______.(a) 29(b) 3(c) 49(d) 2
›Reveal solutionSolution
Integrate with respect to y since the region is naturally bounded by the y-axis and a horizontal line.
From y2=4x, x=4y2. Area =∫03xdy=∫034y2dy=[12y3]03=1227=49.
✓Final answerThe correct option is (c) 49.
- GSEB Higher Secondary Certificate (HSC) Examination 2022Set ANNUAL1 markMCQQ.The area of the parabola y2=12x bounded by its latus rectum is ___.(a) 24(b) 12(c) 18(d) 30
›Reveal solutionSolution
Integrate the parabola up to its latus rectum x=a=3 and double for symmetry.
y2=12x⇒4a=12⇒a=3; latus rectum is at x=3.
Area =2∫0312xdx=2⋅23∫03x1/2dx=43[32x3/2]03.
=43⋅32⋅33=43⋅23=8⋅3=24.
✓Final answer(a) 24.
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