arthham

JEE Advanced 2018 · Integrals

For each positive integer nn, let yn=[1n(n+1)(n+2)…(n+n)]1ny_n = \left[\frac{1}{n}(n+1)(n+2)\ldots(n+n)\right]^{\frac{1}{n}} For x∈Rx \in \mathbb{R}, let [x][x] be the greatest integer less than or equal to xx. If lim⁡n→∞yn=L\lim_{n\to\infty} y_n = L, then the value of [L][L] is ______.

Answer: 11

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

For each positive integer n, let yₙ = [1/n(n+1)(n+2)…(n+n)]^(1/n) For x ∈ R, let [x] be the greatest integer less than or equal to x. If lim_(n→∞) yₙ = L, then the value of [L] is ______.

yn=[1nn(n+1)(n+2)⋯(n+n)]1/ny_n = \left[\dfrac{1}{n^n}(n+1)(n+2)\cdots(n+n)\right]^{1/n}

[x]=greatest integer≤x[x] = \text{greatest integer} \le x

To find: [L],L=lim⁡n→∞yn\text{To find: } [L],\qquad L = \lim_{n\to\infty} y_n

(n+1)(n+2)⋯(n+n)nn=∏k=1n(1+kn)\dfrac{(n+1)(n+2)\cdots(n+n)}{n^n} = \prod_{k=1}^{n}\left(1+\dfrac{k}{n}\right)

yn=[∏k=1n(1+kn)]1/ny_n = \left[\prod_{k=1}^{n}\left(1+\dfrac{k}{n}\right)\right]^{1/n}

ln⁡yn=1n∑k=1nln⁡ ⁣(1+kn)\ln y_n = \dfrac{1}{n}\sum_{k=1}^{n}\ln\!\left(1+\dfrac{k}{n}\right)

lim⁡n→∞ln⁡yn=∫01ln⁡(1+x) dx\lim_{n\to\infty}\ln y_n = \int_0^1 \ln(1+x)\,dx

∫ln⁡(1+x) dx=(1+x)ln⁡(1+x)−(1+x)\int \ln(1+x)\,dx = (1+x)\ln(1+x) - (1+x)

∫01=(2ln⁡2−2)−(0−1)=2ln⁡2−1\int_0^1 = (2\ln2 - 2) - (0 - 1) = 2\ln2 - 1

ln⁡L=2ln⁡2−1=ln⁡4e\ln L = 2\ln2 - 1 = \ln\dfrac{4}{e}

L=4e≈1.47L = \dfrac{4}{e} \approx 1.47

[L]=1\boxed{[L] = 1}

More from Integrals

All 55 Integrals questions →

Integrals: formulas and what the chapter covers →