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Area Under the Curves question

2017 · Shift 0 · Q38
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  5. /2017 · Shift 0 · Q38

Area Under the Curves question

2017 · Shift 0 · Q38

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area (in sq. units) of the region {(x,y):x≥0,x+y≤3,x2≤4y and y≤1+x}\left\{ {\left( {x,y} \right):x \ge 0,x + y \le 3,{x^2} \le 4y\,and\,y \le 1 + \sqrt x } \right\}{(x,y):x≥0,x+y≤3,x2≤4yandy≤1+x​} is
  1. A
    32{3 \over 2}23​
  2. B
    73{7 \over 3}37​
  3. C
    52{5 \over 2}25​
  4. D
    5912{59 \over 12}1259​
View written solutionFree

Correct answer: C

  1. Interpret the region

We need the area of the set

{(x,y):x≥0, x+y≤3, x2≤4y, y≤1+x}.\{(x,y): x\ge 0,\ x+y\le 3,\ x^2\le 4y,\ y\le 1+\sqrt{x}\}.{(x,y):x≥0, x+y≤3, x2≤4y, y≤1+x​}.

Rewrite each inequality in a more useful form:

  • x≥0x\ge 0x≥0
  • x+y≤3  ⟹  y≤3−xx+y\le 3 \implies y\le 3-xx+y≤3⟹y≤3−x
  • x2≤4y  ⟹  y≥x24x^2\le 4y \implies y\ge \dfrac{x^2}{4}x2≤4y⟹y≥4x2​
  • y≤1+xy\le 1+\sqrt{x}y≤1+x​

So for each x≥0x\ge 0x≥0, the region lies above y=x24y=\frac{x^2}{4}y=4x2​ and below both y=3−xandy=1+x.y=3-x \quad \text{and} \quad y=1+\sqrt{x}.y=3−xandy=1+x​.

Hence upper boundary is min⁡(3−x, 1+x).\min(3-x,\ 1+\sqrt{x}).min(3−x, 1+x​).

So area is determined by comparing the two upper curves.


  1. Find intersection of the two upper curves

Solve

3−x=1+x.3-x = 1+\sqrt{x}.3−x=1+x​.

Then

2−x=x.2-x = \sqrt{x}.2−x=x​.

Let t=xt=\sqrt{x}t=x​, so x=t2x=t^2x=t2 with t≥0t\ge 0t≥0. Then

2−t2=t2-t^2=t2−t2=t t2+t−2=0t^2+t-2=0t2+t−2=0 (t+2)(t−1)=0.(t+2)(t-1)=0.(t+2)(t−1)=0.

Since t≥0t\ge 0t≥0, we get t=1t=1t=1, hence

At x=1x=1x=1, both give y=2y=2y=2.

Now check which is smaller:

  • At x=0x=0x=0: 3−x=33-x=33−x=3, 1+x=11+\sqrt{x}=11+x​=1, so 1+x1+\sqrt{x}1+x​ is smaller.
  • For x>1x>1x>1, 3−x3-x3−x becomes smaller.

Thus,

  • for 0≤x≤10\le x\le 10≤x≤1, upper curve is y=1+xy=1+\sqrt{x}y=1+x​,
  • for x≥1x\ge 1x≥1, upper curve is y=3−xy=3-xy=3−x.

  1. Find where the lower curve meets the relevant upper curves

The lower curve is y=x24.y=\frac{x^2}{4}.y=4x2​.

(i) Intersection with y=1+xy=1+\sqrt{x}y=1+x​

Solve

x24=1+x.\frac{x^2}{4}=1+\sqrt{x}.4x2​=1+x​.

Let t=xt=\sqrt{x}t=x​, so x=t2x=t^2x=t2. Then

t44=1+t\frac{t^4}{4}=1+t4t4​=1+t t4=4t+4.t^4=4t+4.t4=4t+4.

Check t=2t=\sqrt{2}t=2​: t4=4,4t+4>4,t^4=4, \quad 4t+4>4,t4=4,4t+4>4, so not equal. Instead test x=4x=4x=4:

so intersection is not in the interval near [0,1][0,1][0,1]. Since on 0≤x≤10\le x\le 10≤x≤1, we have x24≤14<1+x,\frac{x^2}{4}\le \frac14 < 1+\sqrt{x},4x2​≤41​<1+x​, there is no issue there; region exists throughout [0,1][0,1][0,1].

(ii) Intersection with y=3−xy=3-xy=3−x

Solve

x24=3−x.\frac{x^2}{4}=3-x.4x2​=3−x.

Multiply by 444:

x2=12−4xx^2=12-4xx2=12−4x x2+4x−12=0x^2+4x-12=0x2+4x−12=0 (x+6)(x−2)=0.(x+6)(x-2)=0.(x+6)(x−2)=0.

Since x≥0x\ge 0x≥0, we get

Thus for x>2x>2x>2, the lower curve exceeds 3−x3-x3−x, so no region exists.

Hence the total region spans:

  • 0≤x≤10\le x\le 10≤x≤1: between y=x24y=\dfrac{x^2}{4}y=4x2​ and y=1+xy=1+\sqrt{x}y=1+x​
  • 1≤x≤21\le x\le 21≤x≤2: between y=x24y=\dfrac{x^2}{4}y=4x2​ and y=3−xy=3-xy=3−x

  1. Set up the area integral

Therefore,

A=∫01(1+x−x24)dx+∫12(3−x−x24)dx.A=\int_0^1 \left(1+\sqrt{x}-\frac{x^2}{4}\right)dx +\int_1^2 \left(3-x-\frac{x^2}{4}\right)dx.A=∫01​(1+x​−4x2​)dx+∫12​(3−x−4x2​)dx.
  1. Evaluate the first integral
I1=∫01(1+x−x24)dxI_1=\int_0^1 \left(1+\sqrt{x}-\frac{x^2}{4}\right)dxI1​=∫01​(1+x​−4x2​)dx

Integrate termwise:

∫1 dx=x,∫x dx=23x3/2,∫x24 dx=x312.\int 1\,dx=x, \qquad \int \sqrt{x}\,dx=\frac{2}{3}x^{3/2}, \qquad \int \frac{x^2}{4}\,dx=\frac{x^3}{12}.∫1dx=x,∫x​dx=32​x3/2,∫4x2​dx=12x3​.

So

I1=[x+23x3/2−x312]01=1+23−112.I_1=\left[x+\frac{2}{3}x^{3/2}-\frac{x^3}{12}\right]_0^1 =1+\frac23-\frac1{12}.I1​=[x+32​x3/2−12x3​]01​=1+32​−121​.

Compute:

1+\frac23-\frac1{12}= rac{12+8-1}{12}= rac{19}{12}.
  1. Evaluate the second integral
I2=∫12(3−x−x24)dxI_2=\int_1^2 \left(3-x-\frac{x^2}{4}\right)dxI2​=∫12​(3−x−4x2​)dx

Integrate termwise:

∫(3−x−x24)dx=3x−x22−x312.\int \left(3-x-\frac{x^2}{4}\right)dx =3x-\frac{x^2}{2}-\frac{x^3}{12}.∫(3−x−4x2​)dx=3x−2x2​−12x3​.

Thus

I2=[3x−x22−x312]12.I_2=\left[3x-\frac{x^2}{2}-\frac{x^3}{12}\right]_1^2.I2​=[3x−2x2​−12x3​]12​.

At x=2x=2x=2:

3(2)−42−812=6−2−23=103.3(2)-\frac{4}{2}-\frac{8}{12}=6-2-\frac23=\frac{10}{3}.3(2)−24​−128​=6−2−32​=310​.

At x=1x=1x=1:

3−12−112=3−612−112=3−712=2912.3-\frac12-\frac1{12}=3-\frac{6}{12}-\frac{1}{12}=3-\frac{7}{12}=\frac{29}{12}.3−21​−121​=3−126​−121​=3−127​=1229​.

Therefore,

I_2=\frac{10}{3}-\frac{29}{12}= rac{40-29}{12}=\frac{11}{12}.
  1. Total area
A=I1+I2=1912+1112=3012=52.A=I_1+I_2=\frac{19}{12}+\frac{11}{12}=\frac{30}{12}=\frac52.A=I1​+I2​=1219​+1211​=1230​=25​.

So the required area is

52.\boxed{\frac52}.25​​.
  1. Check options
  • A: 32\frac3223​ ❌
  • B: 73\frac7337​ ❌
  • C: 52\frac5225​ ✅
  • D: 5912\frac{59}{12}1259​ ❌

Therefore, the correct option is C.

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