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Area Under the Curves question

2023 · 30 Jan · Shift 1 · Q36
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Area Under the Curves question

2023 · 30 Jan · Shift 1 · Q36

JEE MainMathematicsArea Under the CurvesNumerical+4 / −1
Let α\alphaα be the area of the larger region bounded by the curve y2=8xy^{2}=8 xy2=8x and the lines y=xy=xy=x and x=2x=2x=2, which lies in the first quadrant. Then the value of 3α3 \alpha3α is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 22

  1. Given curves and lines

We have:

  • Parabola: y2=8xy^2=8xy2=8x
  • Line: y=xy=xy=x
  • Vertical line: x=2x=2x=2

We want the larger bounded region in the first quadrant.


  1. Find intersection points

(i) Intersection of y=xy=xy=x and y2=8xy^2=8xy2=8x

Substitute y=xy=xy=x into y2=8xy^2=8xy2=8x:

x2=8xx^2=8xx2=8x x(x−8)=0x(x-8)=0x(x−8)=0

So, x=0x=0x=0 or x=8x=8x=8. Since y=xy=xy=x, intersection points are:

(0,0),(8,8)(0,0), \quad (8,8)(0,0),(8,8)

(ii) Intersection of x=2x=2x=2 with the parabola

From y2=8xy^2=8xy2=8x:

y2=16⇒y=4y^2=16 \Rightarrow y=4y2=16⇒y=4

(first quadrant), so point is (2,4)(2,4)(2,4).

(iii) Intersection of x=2x=2x=2 with y=xy=xy=x

This gives point (2,2)(2,2)(2,2).


  1. Identify the bounded region in the first quadrant

Between x=2x=2x=2 and x=8x=8x=8, the line y=xy=xy=x and the upper branch of parabola

y=8xy=\sqrt{8x}y=8x​

form a closed region together with the line x=2x=2x=2.

At x=2x=2x=2:

  • line gives y=2y=2y=2
  • parabola gives y=4y=4y=4

So the enclosed region has vertical slice height:

upper−lower=8x−x\text{upper} - \text{lower} = \sqrt{8x}-xupper−lower=8x​−x

for 2≤x≤82 \le x \le 82≤x≤8.

This is the larger bounded region.


  1. Compute the area α\alphaα

α=∫28(8x−x) dx\alpha=\int_2^8 (\sqrt{8x}-x)\,dxα=∫28​(8x​−x)dx

Now,

8x=22 x1/2\sqrt{8x}=2\sqrt{2}\,x^{1/2}8x​=22​x1/2

Hence,

α=∫2822x1/2 dx−∫28x dx\alpha=\int_2^8 2\sqrt{2}x^{1/2}\,dx - \int_2^8 x\,dxα=∫28​22​x1/2dx−∫28​xdx

Compute each integral:

∫22x1/2 dx=22⋅23x3/2=423x3/2\int 2\sqrt{2}x^{1/2}\,dx = 2\sqrt{2}\cdot \frac{2}{3}x^{3/2}=\frac{4\sqrt{2}}{3}x^{3/2}∫22​x1/2dx=22​⋅32​x3/2=342​​x3/2

∫x dx=x22\int x\,dx=\frac{x^2}{2}∫xdx=2x2​

So,

α=[423x3/2−x22]28\alpha=\left[\frac{4\sqrt{2}}{3}x^{3/2}-\frac{x^2}{2}\right]_2^8α=[342​​x3/2−2x2​]28​

Now evaluate:

For x=8x=8x=8:

83/2=88=8⋅22=1628^{3/2}=8\sqrt{8}=8\cdot 2\sqrt{2}=16\sqrt{2}83/2=88​=8⋅22​=162​

Thus,

423⋅162=4⋅16⋅23=1283\frac{4\sqrt{2}}{3}\cdot 16\sqrt{2}=\frac{4\cdot 16\cdot 2}{3}=\frac{128}{3}342​​⋅162​=34⋅16⋅2​=3128​

and

822=32\frac{8^2}{2}=32282​=32

So value at x=8x=8x=8 is

1283−32=128−963=323\frac{128}{3}-32=\frac{128-96}{3}=\frac{32}{3}3128​−32=3128−96​=332​

For x=2x=2x=2:

23/2=222^{3/2}=2\sqrt{2}23/2=22​

Thus,

423⋅22=4⋅2⋅23=163\frac{4\sqrt{2}}{3}\cdot 2\sqrt{2}=\frac{4\cdot 2\cdot 2}{3}=\frac{16}{3}342​​⋅22​=34⋅2⋅2​=316​

and

222=2\frac{2^2}{2}=2222​=2

So value at x=2x=2x=2 is

163−2=16−63=103\frac{16}{3}-2=\frac{16-6}{3}=\frac{10}{3}316​−2=316−6​=310​

Therefore,

α=323−103=223\alpha=\frac{32}{3}-\frac{10}{3}=\frac{22}{3}α=332​−310​=322​


  1. Find 3α3\alpha3α

3α=3⋅223=223\alpha=3\cdot \frac{22}{3}=223α=3⋅322​=22


  1. Comparison with stored answer

Stored correct answer = 222222.

Our derived answer is also 222222, so it agrees.

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