arthham

JEE Advanced 2022 · Integrals

For positive integer nn, define f(n)=n+16+5n−3n24n+3n2+32+n−3n28n+3n2+48−3n−3n212n+3n2+...+25n−7n27n2f(n) = n + \frac{16+5n-3n^{2}}{4n+3n^{2}} + \frac{32+n-3n^{2}}{8n+3n^{2}} + \frac{48-3n-3n^{2}}{12n+3n^{2}} + ... + \frac{25n-7n^{2}}{7n^{2}}. Then, the value of lim⁡n→∞f(n)\lim_{n \to \infty} f(n) is equal to

Answer: B. 4−34log⁡e(73)4 - \frac{3}{4}\log_{e}\left(\frac{7}{3}\right)

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 positive integer n, define f(n) = n + (16+5n-3n²)/(4n+3n²) + (32+n-3n²)/(8n+3n²) + (48-3n-3n²)/(12n+3n²) + ... + (25n-7n²)/7n². Then, the value of lim_(n → ∞) f(n) is equal to f(n)=n+16+5n−3n24n+3n2+32+n−3n28n+3n2+…+25n−7n27n2f(n) = n + \dfrac{16+5n-3n^2}{4n+3n^2} + \dfrac{32+n-3n^2}{8n+3n^2} + \ldots + \dfrac{25n-7n^2}{7n^2} lim⁡n→∞f(n)=?\lim_{n\to\infty} f(n) = ? Tk=16k+(9−4k)n−3n24kn+3n2,k=1,2,…,nT_k = \dfrac{16k + (9-4k)n - 3n^2}{4kn + 3n^2},\qquad k = 1, 2, \ldots, n k=n: 16n+(9−4n)n−3n24n2+3n2=25n−7n27n2 ✓k=n:\ \dfrac{16n + (9-4n)n - 3n^2}{4n^2 + 3n^2} = \dfrac{25n - 7n^2}{7n^2}\ \checkmark Tk=−1+16k+9n4kn+3n2=−1+16k+9nn(3n+4k)T_k = -1 + \dfrac{16k+9n}{4kn+3n^2} = -1 + \dfrac{16k+9n}{n(3n+4k)} f(n)=n+∑k=1nTk=n−n+∑k=1n16k+9nn(3n+4k)f(n) = n + \sum_{k=1}^{n} T_k = n - n + \sum_{k=1}^{n} \dfrac{16k+9n}{n(3n+4k)} =∑k=1n16k+9nn(3n+4k)= \sum_{k=1}^{n} \dfrac{16k+9n}{n(3n+4k)} =1n∑k=1n16kn+93+4kn=1n∑k=1ng ⁣(kn)= \dfrac1n \sum_{k=1}^{n} \dfrac{16\frac{k}{n}+9}{3+4\frac{k}{n}} = \dfrac1n\sum_{k=1}^{n} g\!\left(\dfrac{k}{n}\right) g(x)=16x+93+4xg(x) = \dfrac{16x+9}{3+4x} lim⁡n→∞f(n)=∫0116x+93+4x dx\lim_{n\to\infty} f(n) = \displaystyle\int_0^1 \dfrac{16x+9}{3+4x}\,dx 16x+93+4x=4−34x+3\dfrac{16x+9}{3+4x} = 4 - \dfrac{3}{4x+3} ∫01 ⁣(4−34x+3)dx=[4x−34ln⁡(4x+3)]01\int_0^1\!\left(4 - \dfrac{3}{4x+3}\right)dx = \left[4x - \dfrac34\ln(4x+3)\right]_0^1 =4−34ln⁡ ⁣(73)= 4 - \dfrac34\ln\!\left(\dfrac73\right) 4−34log⁡e ⁣(73)(2)\boxed{4 - \dfrac34\log_e\!\left(\dfrac73\right)} \quad (2)

More from Integrals

All 55 Integrals questions →

Integrals: formulas and what the chapter covers →