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Area Under the Curves question

2023 · 24 Jan · Shift 1 · Q31
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  5. /2023 · 24 Jan · Shift 1 · Q31

Area Under the Curves question

2023 · 24 Jan · Shift 1 · Q31

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area enclosed by the curves y2+4x=4{y^2} + 4x = 4y2+4x=4 and y−2x=2y - 2x = 2y−2x=2 is :
  1. A
    223{{22} \over 3}322​
  2. B
    9
  3. C
    233{{23} \over 3}323​
  4. D
    253{{25} \over 3}325​
View written solutionFree

Correct answer: B

  1. Write the curves in convenient form

The given curves are:

y2+4x=4y^2+4x=4y2+4x=4 y−2x=2y-2x=2y−2x=2

Rewrite them as xxx in terms of yyy:

  • Parabola: 4x=4−y2  ⟹  x=1−y244x=4-y^2 \implies x=1-\frac{y^2}{4}4x=4−y2⟹x=1−4y2​

  • Line: y−2x=2  ⟹  2x=y−2  ⟹  x=y−22y-2x=2 \implies 2x=y-2 \implies x=\frac{y-2}{2}y−2x=2⟹2x=y−2⟹x=2y−2​


  1. Find points of intersection

At intersection,

1−y24=y−221-\frac{y^2}{4}=\frac{y-2}{2}1−4y2​=2y−2​

Multiply by 444:

4−y2=2y−44-y^2=2y-44−y2=2y−4

8−2y−y2=08-2y-y^2=08−2y−y2=0

y2+2y−8=0y^2+2y-8=0y2+2y−8=0

Factor:

(y+4)(y−2)=0(y+4)(y-2)=0(y+4)(y−2)=0

So,

y=−4,y=2y=-4,\quad y=2y=−4,y=2

Now find corresponding xxx values:

  • For y=2y=2y=2: x=2−22=0x=\frac{2-2}{2}=0x=22−2​=0

  • For y=−4y=-4y=−4: x=−4−22=−3x=\frac{-4-2}{2}=-3x=2−4−2​=−3

Thus intersection points are:

(0,2),  (−3,−4)(0,2),\; (-3,-4)(0,2),(−3,−4)


  1. Decide which curve is on the right

For area using horizontal strips, compare xxx-values:

xparabola=1−y24,xline=y−22x_{\text{parabola}}=1-\frac{y^2}{4}, \qquad x_{\text{line}}=\frac{y-2}{2}xparabola​=1−4y2​,xline​=2y−2​

Take a test value, say y=0y=0y=0:

xparabola=1,xline=−1x_{\text{parabola}}=1, \qquad x_{\text{line}}=-1xparabola​=1,xline​=−1

So the parabola lies to the right of the line.

Hence area is

A=∫−42(xright−xleft)dyA=\int_{-4}^{2}\left(x_{\text{right}}-x_{\text{left}}\right)dyA=∫−42​(xright​−xleft​)dy

A=∫−42(1−y24−y−22)dyA=\int_{-4}^{2}\left(1-\frac{y^2}{4}-\frac{y-2}{2}\right)dyA=∫−42​(1−4y2​−2y−2​)dy

Simplify the integrand:

1−y24−y2+1=2−y2−y241-\frac{y^2}{4}-\frac{y}{2}+1=2-\frac{y}{2}-\frac{y^2}{4}1−4y2​−2y​+1=2−2y​−4y2​

So,

A=∫−42(2−y2−y24)dyA=\int_{-4}^{2}\left(2-\frac{y}{2}-\frac{y^2}{4}\right)dyA=∫−42​(2−2y​−4y2​)dy


  1. Integrate
=2y-\frac{y^2}{4}-\frac{y^3}{12}$$ Therefore, $$A=\left[2y-\frac{y^2}{4}-\frac{y^3}{12}\right]_{-4}^{2}$$ At $y=2$: $$2(2)-\frac{2^2}{4}-\frac{2^3}{12}=4-1-\frac{8}{12}=3-\frac{2}{3}=\frac{7}{3}$$ At $y=-4$: $$2(-4)-\frac{(-4)^2}{4}-\frac{(-4)^3}{12} =-8-4+\frac{64}{12} =-12+\frac{16}{3} =\frac{-36+16}{3}=\frac{-20}{3}$$ Thus, $$A=\frac{7}{3}-\left(-\frac{20}{3}\right)=\frac{27}{3}=9$$ --- 5. **Match with options** The enclosed area is: $$\boxed{9}$$ So the correct option is **B**. --- 6. **Comparison with stored answer** Stored correct answer: **B** Derived answer: **B** They match.
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