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Definite Integration question

2024 · 5 Apr · Shift 2 · Q41
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  5. /2024 · 5 Apr · Shift 2 · Q41

Definite Integration question

2024 · 5 Apr · Shift 2 · Q41

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
Let β(m,n)=∫01xm−1(1−x)n−1 dx, m,n>0\beta(\mathrm{m}, \mathrm{n})=\int\limits_0^1 x^{\mathrm{m}-1}(1-x)^{\mathrm{n}-1} \mathrm{~d} x, \mathrm{~m}, \mathrm{n}\gt 0β(m,n)=0∫1​xm−1(1−x)n−1 dx, m,n>0. If ∫01(1−x10)20 dx=a×β(b,c)\int\limits_0^1\left(1-x^{10}\right)^{20} \mathrm{~d} x=\mathrm{a} \times \beta(\mathrm{b}, \mathrm{c})0∫1​(1−x10)20 dx=a×β(b,c), then 100(a+b+c)100(\mathrm{a}+\mathrm{b}+\mathrm{c})100(a+b+c) equals ‾\underline{\hspace{2cm}}​.
  1. A
    2012
  2. B
    1021
  3. C
    1120
  4. D
    2120
View written solutionFree

Correct answer: D

  1. We need to evaluate
I=∫01(1−x10)20 dxI=\int_0^1 (1-x^{10})^{20}\,dxI=∫01​(1−x10)20dx

in the form

I=a×β(b,c),I=a\times \beta(b,c),I=a×β(b,c),

where

β(m,n)=∫01xm−1(1−x)n−1 dx.\beta(m,n)=\int_0^1 x^{m-1}(1-x)^{n-1}\,dx.β(m,n)=∫01​xm−1(1−x)n−1dx.
  1. Use the substitution
t=x10⇒x=t1/10,t=x^{10} \quad \Rightarrow \quad x=t^{1/10},t=x10⇒x=t1/10,

so that

dx=110t−9/10 dt.dx=\frac{1}{10}t^{-9/10}\,dt.dx=101​t−9/10dt.

Also, when x=0x=0x=0, t=0t=0t=0, and when x=1x=1x=1, t=1t=1t=1.

  1. Substitute into the integral:
I=∫01(1−t)20⋅110t−9/10 dt.I=\int_0^1 (1-t)^{20}\cdot \frac{1}{10}t^{-9/10}\,dt.I=∫01​(1−t)20⋅101​t−9/10dt.

That is,

I=110∫01t−9/10(1−t)20 dt.I=\frac{1}{10}\int_0^1 t^{-9/10}(1-t)^{20}\,dt.I=101​∫01​t−9/10(1−t)20dt.
  1. Compare this with the Beta-function form
β(m,n)=∫01tm−1(1−t)n−1 dt.\beta(m,n)=\int_0^1 t^{m-1}(1-t)^{n-1}\,dt.β(m,n)=∫01​tm−1(1−t)n−1dt.

We identify:

  • m−1=−910m-1=-\frac{9}{10}m−1=−109​, so m=110;m=\frac{1}{10};m=101​;
  • n−1=20n-1=20n−1=20, so n=21.n=21.n=21.

Hence,

I=110β(110,21).I=\frac{1}{10}\beta\left(\frac{1}{10},21\right).I=101​β(101​,21).

Therefore,

a=110,b=110,c=21.a=\frac{1}{10},\quad b=\frac{1}{10},\quad c=21.a=101​,b=101​,c=21.
  1. Now compute
100(a+b+c)=100(110+110+21).100(a+b+c)=100\left(\frac{1}{10}+\frac{1}{10}+21\right).100(a+b+c)=100(101​+101​+21).

Since

110+110=210=15,\frac{1}{10}+\frac{1}{10}=\frac{2}{10}=\frac{1}{5},101​+101​=102​=51​,

we get

100(21+15)=100⋅1065=20⋅106=2120.100\left(21+\frac{1}{5}\right)=100\cdot \frac{106}{5}=20\cdot 106=2120.100(21+51​)=100⋅5106​=20⋅106=2120.
  1. Therefore the correct option is
D: 2120.\boxed{\text{D: }2120}.D: 2120​.
  1. Comparison with stored answer: The stored correct answer is D, which matches our result.
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