Q.Calculate the area under the curve y=2x included between the lines x=0 and x=1.
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:
Area=∫cdg(y)dy.
Always sketch the region first. The sketch tells you the correct limits, whether the curve dips below the axis, and whether it is cleaner to integrate in x or in y.
The single big idea: any area with a curved boundary is the sum of infinitely many thin strips, and that sum is precisely a definite integral.
Students searching "Area Under Curve formula and examples" or "Application of Integrals class 12 important questions" will find this the core idea tested throughout NCERT's Application of Integrals chapter, a mainstay of the CBSE Class 12 Maths syllabus and JEE Main/Advanced. Mastering the sign convention for regions below the x-axis is one of the most frequently asked concepts in board and competitive exam papers alike.
Concept: Area Under Curve — the area bounded by y=f(x), the x-axis, and the vertical lines x=a, x=b is given by ∫abf(x)dx.
Step 1: Identify the curve and limits.
y=2x, from x=0 to x=1. The curve lies above the x-axis in this interval.
Step 2: Set up the definite integral.
Area=∫012xdx
Step 3: Integrate.
Recall x=x1/2, so
∫2x1/2dx=2⋅3/2x3/2=2⋅32x3/2=34x3/2
Step 4: Evaluate from 0 to 1.
34(1)3/2−34(0)3/2=34
The area is 34 square units.
The area under y=2x from x=0 to x=1 is found by integrating the function over that interval. The result is 34 square units.
Why integration works here
When we talk about "area under a curve" between two vertical lines, we mean the region bounded by the curve y=f(x), the x-axis, and the lines x=a and x=b. The fundamental idea is that we slice this region into infinitely thin vertical strips of width dx and height f(x). The area of each strip is f(x)dx, and adding them all up gives the definite integral ∫abf(x)dx.
For y=2x, the curve lies entirely above the x-axis for x≥0, so no sign issues arise — the integral directly gives the geometric area.
Area under y=f(x) from x=a to x=b is ∫abf(x)dx, provided f(x)≥0 on [a,b].
Step-by-step calculation
1. Set up the integral.
The boundaries are x=0 and x=1, and the function is y=2x. So the area A is:
A=∫012xdx
2. Rewrite the integrand in power form.
Recall that x=x1/2. So:
A=∫012x1/2dx
3. Apply the power rule for integration.
For any n=−1, ∫xndx=n+1xn+1+C. Here n=21, so n+1=23:
∫2x1/2dx=2⋅3/2x3/2=2⋅32x3/2=34x3/2
A quick check: differentiating 34x3/2 gives 34⋅23x1/2=2x1/2, which matches the original integrand. Always verify your antiderivative if time permits.
4. Evaluate the definite integral.
Using the Fundamental Theorem of Calculus:
A=[34x3/2]01=34(1)3/2−34(0)3/2=34⋅1−0=34
A common mistake is forgetting that x3/2 at x=0 is 0, not undefined. Since 3/2>0, the expression is perfectly well-defined at zero. Also, don't confuse x with x2 — the power rule works the same way, but the exponent matters.
The area is 34 square units.
Method: Area under a single curve between two vertical lines
Use this whenever the region is bounded above by one curve y=f(x), below by the x-axis, and on the sides by x=a and x=b, with f(x)≥0 throughout.
Steps
Step 1: Confirm the curve stays above the axis.
On the interval [a,b], check that f(x)≥0 (for roots and even powers this is usually automatic). If it dips below, you would need ∣f(x)∣ — so this check protects the sign of the answer.
Step 2: Write the area as a definite integral.
A=∫abf(x)dx.
This simply sums thin vertical strips of height f(x) and width dx.
Step 3: Rewrite roots as fractional powers, then apply the power rule.
Convert any radical, e.g. x=x1/2, and integrate term by term with
∫xndx=n+1xn+1+C(n=−1).
Step 4: Substitute the limits (upper minus lower).
Evaluate the antiderivative at b and a and subtract. A quick verification is to differentiate your antiderivative and confirm it returns the original integrand.
Common Mistakes
Mistake 1: Dropping the coefficient 2 in y=2x
Why it's wrong: integrating x alone gives 32x3/2 and a final area of 32, but the height of every strip is 2x, not x. Correct approach: keep the factor, ∫012x1/2dx=34x3/201=34.
Mistake 2: Misapplying the power rule to x1/2
Why it's wrong: students write ∫x1/2dx=3x3/2 or forget to raise the exponent by one. Correct approach: with n=21, ∫x1/2dx=3/2x3/2=32x3/2.
Showing the 12 most recent of 16 on this concept.
- 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
∫12(x−1)dx=[2(x−1)2]12=21−0=21 sq. units.
✓Final answerThe correct option is (C).
- KEAM 2021Set eng-2021-P2-B14 marksMCQQ.The area of the region bounded by the curves y=x2 and y=x is (in square units) (A) 32 (B) 31 (C) 61 (D) 65 (E) 1
›Reveal solutionSolution
The enclosed area between y=x2 and y=x is 31.
Concept and Intuition
Between their intersection points, the upper curve minus the lower curve integrated gives the area. On [0,1], x≥x2.
Step-by-Step Solution
- Intersections: x2=x⇒x4=x⇒x=0,1.
- Area =∫01(x−x2)dx.
- =[32x3/2−3x3]01=32−31=31.
Common Mistakes
- Taking x2 as the upper curve on [0,1].
✓Final answerThe correct option is (B) — 31.
ANSWER: B
- 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
Area=∫02(2x−x2)dx=[x2−3x3]02=4−38=34.
✓Final answerThe correct option is (C).
- 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
A=∫02(2x−x2)dx=[x2−3x3]02=4−38=34.
✓Final answerThe correct option is (B).
- 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.
(Equivalently, a trapezium with parallel sides 2 and 3, width 1: area =21(2+3)(1)=25.)
✓Final answerThe correct option is (B).
- KEAM 2022Set eng-2022-P2-B14 marksMCQQ.The area bounded by the curve y=x(2−x) and the line y=x is (A) 61 (B) 31 (C) 21 (D) 65 (E) 32
›Reveal solutionSolution
The enclosed area is 61.
Concept and Intuition
Find the intersection points, determine which curve is on top, then integrate the difference between them.
Step-by-Step Solution
- Set x(2−x)=x⇒2x−x2=x⇒x−x2=0⇒x=0,1.
- At x=0.5: parabola =0.75, line =0.5, so parabola is above.
- Area =∫01[(2x−x2)−x]dx=∫01(x−x2)dx.
- =[2x2−3x3]01=21−31=61.
Common Mistakes
- Choosing the wrong upper curve.
- Integrating x(2−x) without subtracting the line.
✓Final answerThe correct option is (A) — 61.
ANSWER: A
- 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.
At x=4: 32⋅8+216=316+8=316+324=340 square units.
✓Final answerThe correct option is (C).
- 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.
- = [x^2/2 - x^{7/2}/(7/2)] from 0 to 1 = 1/2 - 2/7.
- = 7/14 - 4/14 = 3/14.
Common Mistakes
- Integrating x^{7/2} without dividing by 7/2 (i.e. forgetting the factor 2/7).
✓Final answerThe correct option is (C) — 3/14.
ANSWER: C
- 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.
At x=2: 38+4−2=314. At x=1: 31+2−21=611.
A=314−611=628−11=617.
✓Final answerThe correct option is (D).
- KEAM 2023Set eng-2023-P2-B24 marksMCQQ.The area of the region in the first quadrant enclosed by the curves y=x,y=−x+6 and the x-axis is (A) 722 (B) 322 (C) 12 (D) 24 (E) 8
›Reveal solutionSolution
The area is 322.
Concept and Intuition
The region is bounded on the left by y=x, on the right by the line y=−x+6, and below by the x-axis; split the x-integration at their intersection.
Step-by-Step Solution
- Intersection of y=x and y=−x+6: x=6−x⇒x=4,y=2; the line meets the x-axis at x=6.
- ∫04xdx=[32x3/2]04=316.
- ∫46(−x+6)dx=[−2x2+6x]46=18−16=2; total =316+2=322.
Common Mistakes
- Not splitting at x=4.
- Using the wrong upper limit for the line region.
✓Final answerThe correct option is (B) — 322.
ANSWER: B
- 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
∫04(x3−8x2+16x)dx=[4x4−38x3+8x2]04=64−3512+128=192−3512=364.
✓Final answerThe correct option is (C).
- KEAM 2021Set eng-2021-P2-B14 marksMCQQ.The area of the region bounded by y=5x, x-axis and x=4 is (in square units) (A) 40 (B) 80 (C) 20 (D) 50 (E) 60
›Reveal solutionSolution
The area bounded by y=5x, the x-axis and x=4 is 40 square units.
Concept and Intuition
The area under a line above the x-axis is the definite integral, equivalently the area of a triangle with base 4 and height 5⋅4=20.
Step-by-Step Solution
- Area =∫045xdx=25x204.
- =25⋅16=40.
- Check via triangle: 21⋅4⋅20=40. \checkmark
Common Mistakes
- Forgetting the height is 5⋅4=20, not 4.
✓Final answerThe correct option is (A) — 40.
ANSWER: A
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