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 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 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 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 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 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: …
- 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. …
- 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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