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Definite Integration question

2022 · 29 Jun · Shift 2 · Q28
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  5. /2022 · 29 Jun · Shift 2 · Q28

Definite Integration question

2022 · 29 Jun · Shift 2 · Q28

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
If ∫02(2x−2x−x2)dx=∫01(1−1−y2−y22)dy+∫12(2−y22)dy+I\int\limits_0^2 {\left( {\sqrt {2x} - \sqrt {2x - {x^2}} } \right)dx = \int\limits_0^1 {\left( {1 - \sqrt {1 - {y^2}} - {{{y^2}} \over 2}} \right)dy + \int\limits_1^2 {\left( {2 - {{{y^2}} \over 2}} \right)dy + I} } }0∫2​(2x​−2x−x2​)dx=0∫1​(1−1−y2​−2y2​)dy+1∫2​(2−2y2​)dy+I, then I equals
  1. A
    ∫01(1+1−y2)dy\int\limits_0^1 {\left( {1 + \sqrt {1 - {y^2}} } \right)dy}0∫1​(1+1−y2​)dy
  2. B
    ∫01(y22−1−y2+1)dy\int\limits_0^1 {\left( {{{{y^2}} \over 2} - \sqrt {1 - {y^2}} + 1} \right)dy}0∫1​(2y2​−1−y2​+1)dy
  3. C
    ∫01(1−1−y2)dy\int\limits_0^1 {\left( {1 - \sqrt {1 - {y^2}} } \right)dy}0∫1​(1−1−y2​)dy
  4. D
    ∫01(y22+1−y2+1)dy\int\limits_0^1 {\left( {{{{y^2}} \over 2} + \sqrt {1 - {y^2}} + 1} \right)dy}0∫1​(2y2​+1−y2​+1)dy
View written solutionFree

Correct answer: C

  1. Start with the given integral
L=\int_0^2\left(\sqrt{2x}-\sqrt{2x-x^2}\right)dx

We simplify each term:

2x−x2=1−(x−1)2\sqrt{2x-x^2}=\sqrt{1-(x-1)^2}2x−x2​=1−(x−1)2​

because

2x−x2=1−(x−1)2.2x-x^2=1-(x-1)^2.2x−x2=1−(x−1)2.

So,

L=∫022x dx−∫021−(x−1)2 dx.L=\int_0^2 \sqrt{2x}\,dx-\int_0^2 \sqrt{1-(x-1)^2}\,dx.L=∫02​2x​dx−∫02​1−(x−1)2​dx.
  1. Substitute y=2xy=\sqrt{2x}y=2x​ in the first integral

Then

y=2x  ⟹  x=y22,dx=y dy.y=\sqrt{2x}\implies x=\frac{y^2}{2},\qquad dx=y\,dy.y=2x​⟹x=2y2​,dx=ydy.

When x=0x=0x=0, y=0y=0y=0; when x=2x=2x=2, y=2y=2y=2.

Hence

∫022x dx=∫02y⋅y dy=∫02y2 dy.\int_0^2 \sqrt{2x}\,dx=\int_0^2 y\cdot y\,dy=\int_0^2 y^2\,dy.∫02​2x​dx=∫02​y⋅ydy=∫02​y2dy.

But this is not directly matching the given RHS form, so let us instead convert the whole integral carefully in terms of yyy using the substitution

y=x−1.y=x-1.y=x−1.

For the second term,

∫021−(x−1)2 dx=∫−111−y2 dy=2∫011−y2 dy.\int_0^2\sqrt{1-(x-1)^2}\,dx=\int_{-1}^1\sqrt{1-y^2}\,dy=2\int_0^1\sqrt{1-y^2}\,dy.∫02​1−(x−1)2​dx=∫−11​1−y2​dy=2∫01​1−y2​dy.

Also,

∫022x dx=∫02y2 dy=83.\int_0^2\sqrt{2x}\,dx=\int_0^2 y^2\,dy=\frac{8}{3}.∫02​2x​dx=∫02​y2dy=38​.

So

L=83−2∫011−y2 dy.L=\frac{8}{3}-2\int_0^1\sqrt{1-y^2}\,dy.L=38​−2∫01​1−y2​dy.
  1. Now simplify the given right-hand side except for III

Given

L=∫01(1−1−y2−y22)dy+∫12(2−y22)dy+I.L=\int_0^1\left(1-\sqrt{1-y^2}-\frac{y^2}{2}\right)dy+\int_1^2\left(2-\frac{y^2}{2}\right)dy+I.L=∫01​(1−1−y2​−2y2​)dy+∫12​(2−2y2​)dy+I.

Let

R=∫01(1−1−y2−y22)dy+∫12(2−y22)dy.R=\int_0^1\left(1-\sqrt{1-y^2}-\frac{y^2}{2}\right)dy+\int_1^2\left(2-\frac{y^2}{2}\right)dy.R=∫01​(1−1−y2​−2y2​)dy+∫12​(2−2y2​)dy.

Compute the second integral:

∫12(2−y22)dyn=[2y−y36]12=(4−86)−(2−16)=56.\int_1^2\left(2-\frac{y^2}{2}\right)dy n=\left[2y-\frac{y^3}{6}\right]_1^2 =\left(4-\frac{8}{6}\right)-\left(2-\frac{1}{6}\right) =\frac{5}{6}.∫12​(2−2y2​)dyn=[2y−6y3​]12​=(4−68​)−(2−61​)=65​.

Now,

∫01(1−1−y2−y22)dy=∫011 dy−∫011−y2 dy−12∫01y2dy\int_0^1\left(1-\sqrt{1-y^2}-\frac{y^2}{2}\right)dy =\int_0^1 1\,dy-\int_0^1\sqrt{1-y^2}\,dy-\frac12\int_0^1 y^2dy∫01​(1−1−y2​−2y2​)dy=∫01​1dy−∫01​1−y2​dy−21​∫01​y2dy =1−∫011−y2 dy−12⋅13=56−∫011−y2 dy.=1-\int_0^1\sqrt{1-y^2}\,dy-\frac12\cdot\frac13 =\frac56-\int_0^1\sqrt{1-y^2}\,dy.=1−∫01​1−y2​dy−21​⋅31​=65​−∫01​1−y2​dy.

Therefore,

R=(56−∫011−y2 dy)+56=53−∫011−y2 dy.R=\left(\frac56-\int_0^1\sqrt{1-y^2}\,dy\right)+\frac56 =\frac53-\int_0^1\sqrt{1-y^2}\,dy.R=(65​−∫01​1−y2​dy)+65​=35​−∫01​1−y2​dy.

So

I=L−R.I=L-R.I=L−R.
  1. Compute III

We found

L=83−2∫011−y2 dyL=\frac83-2\int_0^1\sqrt{1-y^2}\,dyL=38​−2∫01​1−y2​dy

and

R=53−∫011−y2 dy.R=\frac53-\int_0^1\sqrt{1-y^2}\,dy.R=35​−∫01​1−y2​dy.

Hence

I=(83−2∫011−y2 dy)−(53−∫011−y2 dy)I=\left(\frac83-2\int_0^1\sqrt{1-y^2}\,dy\right)-\left(\frac53-\int_0^1\sqrt{1-y^2}\,dy\right)I=(38​−2∫01​1−y2​dy)−(35​−∫01​1−y2​dy) I=1−∫011−y2 dy.I=1-\int_0^1\sqrt{1-y^2}\,dy.I=1−∫01​1−y2​dy.

Now write 111 as an integral:

1=∫011 dy.1=\int_0^1 1\,dy.1=∫01​1dy.

Thus

I=∫01(1−1−y2)dy.I=\int_0^1\left(1-\sqrt{1-y^2}\right)dy.I=∫01​(1−1−y2​)dy.
  1. Match with options

This is exactly Option C:

∫01(1−1−y2)dy.\int_0^1\left(1-\sqrt{1-y^2}\right)dy.∫01​(1−1−y2​)dy.

Therefore, the correct answer is C.

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