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 22 on this concept.
- GUJCET 2023Set 091 markMCQQ.Find the area of the region bounded by the line y=3−x, the X-axis and the ordinates x=2 and x=5. (A) 3 (B) 21 (C) 25 (D) 23
›Reveal solutionSolution
The line changes sign at x=3, so split the interval and add the absolute areas.
Concept. Area between a curve and the X-axis is ∫∣y∣dx; the line y=3−x meets the axis at x=3.
Solution.
∫23(3−x)dx=[3x−2x2]23=4.5−4=0.5, …
- GUJCET 2025Set 031 markMCQQ.Area of the region bounded by the curve y=x3, x-axis and the ordinates x=−1 and x=2 is (A) 417 (B) 419 (C) 415 (D) 49
›Reveal solutionSolution
y=x3<0 on (−1,0) and >0 on (0,2), so add the magnitudes of the two parts. …
- GUJCET 2025Set 031 markMCQQ.The area bounded by the curve y=sinx between x=−2π and x=2π is _____. (A) 4 (B) 2 (C) 3 (D) 1
›Reveal solutionSolution
[!TLDR]
Series current is 0.625 A, and the P-Q drop across the 32 Ω resistor is 20 V.
Concept
In a series potential divider the same current flows through both resistors, and the voltage across each is V=IR (Ohm's law).
Solution
The 64 Ω and 32 Ω resistors are in series between 60 V and 0 V, so the total resistance is
R=64+32=96 Ω,
and the current is …
- GUJCET 2019Set 171 markMCQQ.The area bounded by curve y=sin2x (x=0 to x=π) and X-axis is . (A) 2 (B) 1 (C) 4 (D) 23
›Reveal solutionSolution
Take the magnitude of each half-loop area; total is 2.
Concept. sin2x≥0 on [0,π/2] and ≤0 on [π/2,π]; area uses absolute values.
Steps. …
- GUJCET 2024Set 131 markMCQQ.The area bounded by the curve y=cosx between x=−2π and x=2π is __________. (A) 2 (B) 1 (C) 0 (D) 4
›Reveal solutionSolution
cosx≥0 on [−π/2,π/2], so area =∫−π/2π/2cosxdx.
Concept. Since cosx≥0 throughout this interval, the area equals the plain integral. …
- GUJCET 2020Set 071 markMCQQ.The smallest area enclosed by circle x2+y2=4 and line x+y=2 is ________. (A) π+2 (B) π−2 (C) π (D) 2π
›Reveal solutionSolution
Smaller area = quarter-circle area − triangle area =π−2.
Concept: The circle has radius 2. The line x+y=2 passes through (2,0) and (0,2), cutting off a region in the first quadrant.
- Quarter-circle area (bounded by the two axes and the arc) =41πr2=41π(4)=π. …
- GUJCET 2026Set x1 markMCQQ.The area bounded by the curve y=x∣x∣, X-axis and the ordinates x=−1 and x=1 is ______ (A) 0 (B) 32 (C) 31 (D) 34
›Reveal solutionSolution
By symmetry the area =2∫01x2dx=32.
The curve is y=x∣x∣: for x≥0, y=x2 (above the axis); for x<0, y=−x2 (below the axis). The bounded region between x=−1 and x=1 is symmetric, so the total (unsigned) area is twice the area from 0 to 1: …
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.The area of the region bounded by the curve y=x∣x∣, the x-axis, and the ordinates x=−1 and x=1 is ____.(a) 0(b) 1/3(c) 2/3(d) 4/3
›Reveal solutionSolution
Split the region at x=0 since y=x∣x∣ changes sign, then add both areas.
For x≥0, y=x2; for x<0, y=−x2. Both pieces lie below/above the axis symmetrically, so
…
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.The area of the region bounded by the curve y=sinx between x=0 and x=π is ____.(a) 1(b) 2(c) 3(d) 4
›Reveal solutionSolution
sinx≥0 on [0,π], so directly integrate.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.The area of the region, lying in the first quadrant, bounded by the circle x2+y2=4 and the lines x=0, x=2 = ____.(a) π(b) 3π(c) 2π(d) 4π
›Reveal solutionSolution
This region is exactly the quarter-disc of the circle in the first quadrant.
The circle x2+y2=4 has radius 2. Bounded by x=0 and x=2 in the first quadrant, this describes the full quarter-circle.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.The area of the region bounded by y=sinx, between x=−2π and x=2π = ____.(a) 0(b) 2(c) 1(d) 3
›Reveal solutionSolution
sinx is an odd function, so use symmetry and take absolute value for area (sign changes at x=0).
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.The area of the region bounded by the curve y=x3, the X-axis, and the lines x=−2 and x=1 = ____.(a) −9(b) 415(c) −415(d) 417
›Reveal solutionSolution
y=x3 is negative for x<0 and positive for x>0, so split at x=0 and add the magnitudes.
∫x3dx=4x4.
∫−20x3dx=[4x4]−20=0−4=−4, magnitude 4 (curve is below the axis here).
…
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