Q.Find the area bounded by the lines y=4x+5, y=5−x and 4y=x+5.
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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 Curve — the region enclosed by three lines is a triangle; find its vertices by solving pairwise intersections, then use the shoelace formula or integrate.
Steps:
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Find vertices
Intersection of y=4x+5 and y=5−x:
4x+5=5−x⇒5x=0⇒x=0, so y=5. Vertex A(0,5).
Intersection of y=4x+5 and 4y=x+5:
4(4x+5)=x+5⇒16x+20=x+5⇒15x=−15⇒x=−1, so y=1. Vertex B(−1,1).
Intersection of y=5−x and 4y=x+5:
4(5−x)=x+5⇒20−4x=x+5⇒15=5x⇒x=3, so y=2. Vertex C(3,2).
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Area using shoelace formula …
The three lines meet at (−1,1), (0,5) and (3,2); the enclosed triangle has area 215 (i.e. 7.5) square units.
Concept
Three non-parallel, non-concurrent lines bound a triangle. Find the three pairwise intersection points, then compute the triangle's area (shoelace formula).
Solution
1. y=4x+5 and y=5−x: 4x+5=5−x⇒5x=0⇒x=0, y=5. Point A(0,5).
2. y=4x+5 and 4y=x+5: 4(4x+5)=x+5⇒16x+20=x+5⇒15x=−15⇒x=−1, y=1. Point B(−1,1).
3. y=5−x and 4y=x+5: 4(5−x)=x+5⇒20−4x=x+5⇒5x=15⇒x=3, y=2. Point C(3,2).
4. Shoelace formula with A(0,5), B(−1,1), C(3,2):
Area=21xA(yB−yC)+xB(yC−yA)+xC(yA−yB) …
Method: Area of the triangle cut out by three lines
Three lines that are pairwise non-parallel and do not all pass through one point fence off a triangular region. Find its corners, then compute the area straight from the coordinates.
Steps
Step 1: Locate the three vertices.
A vertex is where two of the lines meet, so take the lines two at a time and solve each pair. Three pairs give three corner points.
Step 2: Feed the corners into the coordinate area formula.
For vertices (x1,y1), (x2,y2), (x3,y3),
A=21∣x1(y2−y3)+x2(y3−y1)+x3(y1−y2)∣.
Step 3: Verify with base × height (optional but recommended). …
Common Mistakes
Mistake 1: Misreading 4y=x+5 as y=x+5.
Why it's wrong: forgetting to divide by 4 changes the line's slope entirely and gives wrong vertices. Correct approach: rewrite it as y=4x+5 (slope 41) before solving any intersection.
Mistake 2: Listing the vertices out of order in the shoelace formula. …
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 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 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-2024-06064 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-2025-04264 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-2025-04294 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 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 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-2025-04284 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 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 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. …
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