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Definite Integration question

2021 · Shift 2 · Q38
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Definite Integration question

2021 · Shift 2 · Q38

JEE AdvancedMathematicsDefinite IntegrationNumerical+4 / −1
For any real number x, let [ x ] denote the largest integer less than or equal to x. If I=∫010[10xx+1]dxI = \int\limits_0^{10} {\left[ {\sqrt {{{10x} \over {x + 1}}} } \right]dx}I=0∫10​[x+110x​​]dx, then the value of 9I is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 173

  1. We need to evaluate I=∫010[10xx+1]dx.I=\int_0^{10}\left[\sqrt{\frac{10x}{x+1}}\right]dx.I=∫010​[x+110x​​]dx. Let f(x)=10xx+1.f(x)=\sqrt{\frac{10x}{x+1}}.f(x)=x+110x​​. Since x∈[0,10]x\in[0,10]x∈[0,10], the quantity 10xx+1\frac{10x}{x+1}x+110x​ increases with xxx, so f(x)f(x)f(x) is increasing.

  2. Find the range of f(x)f(x)f(x) on [0,10][0,10][0,10]: f(0)=0,f(10)=10011<9=3.f(0)=0,\qquad f(10)=\sqrt{\frac{100}{11}}<\sqrt{9}=3.f(0)=0,f(10)=11100​​<9​=3. So 0≤f(x)<3.0\le f(x)<3.0≤f(x)<3. Hence [f(x)]\left[f(x)\right][f(x)] can take only the values 0,1,20,1,20,1,2.

  3. We now find where f(x)f(x)f(x) crosses 111 and 222.

For f(x)≥1f(x)\ge 1f(x)≥1: 10xx+1≥1\sqrt{\frac{10x}{x+1}}\ge 1x+110x​​≥1 10xx+1≥1\frac{10x}{x+1}\ge 1x+110x​≥1 10x≥x+110x\ge x+110x≥x+1 9x≥19x\ge 19x≥1 x≥19.x\ge \frac19.x≥91​. So:

  • [f(x)]=0[f(x)]=0[f(x)]=0 for 0≤x<190\le x<\frac190≤x<91​.

For f(x)≥2f(x)\ge 2f(x)≥2: 10xx+1≥2\sqrt{\frac{10x}{x+1}}\ge 2x+110x​​≥2 10xx+1≥4\frac{10x}{x+1}\ge 4x+110x​≥4 10x≥4x+410x\ge 4x+410x≥4x+4 6x≥46x\ge 46x≥4 x≥23.x\ge \frac23.x≥32​. So:

  • [f(x)]=1[f(x)]=1[f(x)]=1 for 19≤x<23\frac19\le x<\frac2391​≤x<32​,
  • [f(x)]=2[f(x)]=2[f(x)]=2 for 23≤x≤10\frac23\le x\le 1032​≤x≤10.
  1. Therefore, I=∫01/90 dx+∫1/92/31 dx+∫2/3102 dx.I=\int_0^{1/9}0\,dx+\int_{1/9}^{2/3}1\,dx+\int_{2/3}^{10}2\,dx.I=∫01/9​0dx+∫1/92/3​1dx+∫2/310​2dx. Compute each part: ∫1/92/31 dx=23−19=6−19=59,\int_{1/9}^{2/3}1\,dx=\frac23-\frac19=\frac{6-1}{9}=\frac59,∫1/92/3​1dx=32​−91​=96−1​=95​, ∫2/3102 dx=2(10−23)=2⋅283=563.\int_{2/3}^{10}2\,dx=2\left(10-\frac23\right)=2\cdot\frac{28}{3}=\frac{56}{3}.∫2/310​2dx=2(10−32​)=2⋅328​=356​. Hence I=59+563=59+1689=1739.I=\frac59+\frac{56}{3}=\frac59+\frac{168}{9}=\frac{173}{9}.I=95​+356​=95​+9168​=9173​.

  2. So, 9I=173.9I=173.9I=173.

  3. Comparison with stored answer: The stored correct answer is 182182182, but our computed value is 173173173. Thus, the stored answer appears to be incorrect.

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