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Area Under the Curves question

2019 · 11 Jan · Shift 1 · Q41
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Area Under the Curves question

2019 · 11 Jan · Shift 1 · Q41

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area (in sq. units) of the region bounded by the curve x2 = 4y and the straight line x = 4y – 2 is :
  1. A
    34{3 \over 4}43​
  2. B
    54{5 \over 4}45​
  3. C
    78{7 \over 8}87​
  4. D
    98{9 \over 8}89​
View written solutionFree

Correct answer: D

  1. Write both curves in a convenient form

The parabola is x2=4y  ⟹  y=x24.x^2=4y \implies y=\frac{x^2}{4}.x2=4y⟹y=4x2​.

The line is x=4y−2  ⟹  4y=x+2  ⟹  y=x+24.x=4y-2 \implies 4y=x+2 \implies y=\frac{x+2}{4}.x=4y−2⟹4y=x+2⟹y=4x+2​.

So we need the area between y=x24andy=x+24.y=\frac{x^2}{4} \quad \text{and} \quad y=\frac{x+2}{4}.y=4x2​andy=4x+2​.


  1. Find points of intersection

At intersection, x24=x+24.\frac{x^2}{4}=\frac{x+2}{4}.4x2​=4x+2​.

Multiply by 444: x2=x+2.x^2=x+2.x2=x+2.

So, x2−x−2=0.x^2-x-2=0.x2−x−2=0.

Factorizing, (x−2)(x+1)=0.(x-2)(x+1)=0.(x−2)(x+1)=0.

Hence, x=2orx=−1.x=2 \quad \text{or} \quad x=-1.x=2orx=−1.

Now corresponding yyy-values:

  • For x=2x=2x=2: y=224=1.y=\frac{2^2}{4}=1.y=422​=1.
  • For x=−1x=-1x=−1: y=(−1)24=14.y=\frac{(-1)^2}{4}=\frac14.y=4(−1)2​=41​.

Thus the curves intersect at (−1,14)and(2,1).(-1,\tfrac14) \quad \text{and} \quad (2,1).(−1,41​)and(2,1).


  1. Determine which curve is above the other

Take a test value, say x=0x=0x=0.

For the line: y=0+24=12.y=\frac{0+2}{4}=\frac12.y=40+2​=21​.

For the parabola: y=024=0.y=\frac{0^2}{4}=0.y=402​=0.

So in the interval [−1,2][-1,2][−1,2], the line lies above the parabola.

Hence area is A=∫−12(x+24−x24)dx.A=\int_{-1}^{2}\left(\frac{x+2}{4}-\frac{x^2}{4}\right)dx.A=∫−12​(4x+2​−4x2​)dx.


  1. Evaluate the integral

A=14∫−12(x+2−x2) dx.A=\frac14\int_{-1}^{2}(x+2-x^2)\,dx.A=41​∫−12​(x+2−x2)dx.

Antiderivative: ∫(x+2−x2)dx=x22+2x−x33.\int (x+2-x^2)dx=\frac{x^2}{2}+2x-\frac{x^3}{3}.∫(x+2−x2)dx=2x2​+2x−3x3​.

Therefore, A=14[x22+2x−x33]−12.A=\frac14\left[\frac{x^2}{2}+2x-\frac{x^3}{3}\right]_{-1}^{2}.A=41​[2x2​+2x−3x3​]−12​.

Now compute:

At x=2x=2x=2, 222+2(2)−233=2+4−83=6−83=103.\frac{2^2}{2}+2(2)-\frac{2^3}{3}=2+4-\frac{8}{3}=6-\frac{8}{3}=\frac{10}{3}.222​+2(2)−323​=2+4−38​=6−38​=310​.

At x=−1x=-1x=−1, (−1)22+2(−1)−(−1)33=12−2+13=56−2=−76.\frac{(-1)^2}{2}+2(-1)-\frac{(-1)^3}{3}=\frac12-2+\frac13=\frac{5}{6}-2=-\frac{7}{6}.2(−1)2​+2(−1)−3(−1)3​=21​−2+31​=65​−2=−67​.

So,

=\frac14\left(\frac{20}{6}+\frac{7}{6}\right) =\frac14\cdot\frac{27}{6} =\frac{27}{24} =\frac98.$$ --- 5. **Compare with the options** $$\frac98$$ matches **Option D**. --- 6. **Comparison with stored correct answer** Stored correct answer: **D** Our derived answer: **D** So they agree.
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