arthham

JEE Advanced 2021 · Integrals

For any real number xx, let [x][x] denote the largest integer less than or equal to xx. If I=∫010[10xx+1]dxI = \int_{0}^{10}\left[\sqrt{\frac{10x}{x+1}}\right]dx, then the value of 9I9I is -----.

Answer: 182182

Watch this solved on the board The full walkthrough, written out a line at a time with narration explaining each step as it appears. Pause anywhere and ask why a line is there. Free to watch

Or read the worked solution below. Jump to the steps ↓

Written solution

I=∫010[10xx+1]dxI=\int_{0}^{10}\left[\sqrt{\dfrac{10x}{x+1}}\right]dx

Find I, then 9I\text{Find } I, \text{ then } 9I

ϕ(x)=10xx+1\phi(x)=\sqrt{\dfrac{10x}{x+1}}

ϕ(0)=0,ϕ(10)=10011≈3.01\phi(0)=0,\quad \phi(10)=\sqrt{\dfrac{100}{11}}\approx 3.01

∴[ϕ(x)]∈{0,1,2,3}\therefore [\phi(x)] \in \{0,1,2,3\}

ϕ(x)=k⇔10xx+1=k2\phi(x)=k \Leftrightarrow \dfrac{10x}{x+1}=k^{2}

⇒x=k210−k2\Rightarrow x=\dfrac{k^{2}}{10-k^{2}}

k=1: x=19,k=2: x=46=23k=1:\ x=\dfrac{1}{9},\qquad k=2:\ x=\dfrac{4}{6}=\dfrac{2}{3}

k=3: x=91=9,k=4: x=16−6<0 (invalid)k=3:\ x=\dfrac{9}{1}=9,\qquad k=4:\ x=\dfrac{16}{-6}<0\ \text{(invalid)}

[0,19):0;[19,23):1[0,\tfrac{1}{9}): 0;\quad [\tfrac{1}{9},\tfrac{2}{3}): 1

[23,9):2;[9,10]:3[\tfrac{2}{3},9): 2;\quad [9,10]: 3

I=0⋅19+1⋅(23−19)+2⋅(9−23)+3⋅1I = 0\cdot\tfrac{1}{9} + 1\cdot\left(\tfrac{2}{3}-\tfrac{1}{9}\right) + 2\cdot\left(9-\tfrac{2}{3}\right) + 3\cdot 1

=59+503+3=1829= \dfrac{5}{9} + \dfrac{50}{3} + 3 = \dfrac{182}{9}

9I=9⋅1829=1829I = 9\cdot\dfrac{182}{9} = 182

Answer: 182\boxed{\text{Answer: } 182}

More from Integrals

All 55 Integrals questions →

Integrals: formulas and what the chapter covers →