Q.Find the area of the region bounded by the triangle whose vertices are (−1,1), (0,5) and (3,2), using integration.
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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: …
Concept: Area under a curve — the area of a triangle can be found by integrating the difference between the upper and lower boundary lines over the appropriate x-interval.
Step 1 – Equations of the sides
- Side AB (from (−1,1) to (0,5)): slope =0+15−1=4, equation y=4x+5.
- Side BC (from (0,5) to (3,2)): slope =3−02−5=−1, equation y=−x+5.
- Side AC (from (−1,1) to (3,2)): slope =3+12−1=41, equation y=41x+45.
Step 2 – Set up the integrals
The region is split at x=0 because the upper boundary changes.
For −1≤x≤0: upper line is AB (4x+5), lower line is AC (41x+45).
For 0≤x≤3: upper line is BC (−x+5), lower line is AC (41x+45).
Step 3 – Compute
Area=∫−10[(4x+5)−(41x+45)]dx+∫03[(−x+5)−(41x+45)]dx
Simplify each integrand:
First: 4x+5−41x−45=415x+415=415(x+1). …
Split the triangle at x=0, integrate (top − bottom) over each part, and add: the area is 215 (i.e. 7.5) square units.
Concept
The area enclosed by the three sides equals ∫(upper boundary−lower boundary)dx over the x-span. The upper edge switches at the middle vertex, so the integral is split there; the lower edge is a single line throughout.
Solution
1. Equations of the sides (two-point form) for A(−1,1), B(0,5), C(3,2):
- AB: slope 0−(−1)5−1=4⇒y=4x+5
- BC: slope 3−02−5=−1⇒y=−x+5
- AC: slope 3−(−1)2−1=41⇒y=4x+45
2. Boundaries. AC is the lower edge throughout (at x=0, AC gives 1.25 vs AB,BC giving 5). The upper edge is AB on [−1,0] and BC on [0,3].
3. Set up the integrals.
A=∫−10[(4x+5)−(4x+45)]dx+∫03[(−x+5)−(4x+45)]dx.
Simplify the integrands:
=∫−10(415x+415)dx+∫03(−45x+415)dx.
4. Evaluate. …
Method: Area of a triangle by integration (split at the middle vertex)
This technique finds the area of a triangle from its vertices using definite integrals rather than a ready-made formula, exactly as an "using integration" question demands.
Steps
Step 1: Find the equations of the three sides.
From the vertices, use the two-point form to get each side as a line y=mx+c. You will have three such lines.
Step 2: Identify the upper and lower boundaries.
One side runs along the bottom of the triangle for the whole x-span; the other two form the top but switch at the middle vertex. Sort the vertices by their x-coordinates so you know where that switch occurs.
Step 3: Split the integral at the middle vertex's x-coordinate. …
Common Mistakes
Mistake 1: Not splitting the integral at the middle vertex x=0
Why it's wrong: the upper boundary is side AB (y=4x+5) on [−1,0] but switches to side BC (y=−x+5) on [0,3]; using one line for the whole span mis-measures the triangle. Correct approach: integrate (upper − lower) separately over [−1,0] and [0,3] and add.
Mistake 2: Misidentifying the lower boundary
Why it's wrong: side AC (y=4x+45) is the lower edge across the whole base (at x=0 it gives 1.25, below the top value 5); swapping it with a top side flips signs. Correct approach: subtract AC from whichever upper side applies on each subinterval. …
Showing the 12 most recent of 16 on this concept.
- 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 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 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 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 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. …
- 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 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. …
- 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 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. …
- 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 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 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 …
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