Tính giới hạn của dãy số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyDamaaBa % aaleaacaWGUbaabeaakiabg2da9maalaaabaGaaGOmamaaCaaaleqa % baGaaG4maaaakiabgkHiTiaaigdaaeaacaaIYaWaaWbaaSqabeaaca % aIZaaaaOGaey4kaSIaaGymaaaacaGGUaWaaSaaaeaacaaIZaWaaWba % aSqabeaacaaIZaaaaOGaeyOeI0IaaGymaaqaaiaaiodadaahaaWcbe % qaaiaaiodaaaGccqGHRaWkcaaIXaaaaiaac6cacaGGUaGaaiOlaiaa % c6cadaWcaaqaaiaad6gadaahaaWcbeqaaiaaiodaaaGccqGHsislca % aIXaaabaGaamOBamaaCaaaleqabaGaaG4maaaakiabgUcaRiaaigda % aaaaaa!5126! {u_n} = \frac{{{2^3} - 1}}{{{2^3} + 1}}.\frac{{{3^3} - 1}}{{{3^3} + 1}}....\frac{{{n^3} - 1}}{{{n^3} + 1}}\)
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Lời giải:
Báo saiTa có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % WGRbWaaWbaaSqabeaacaaIZaaaaOGaeyOeI0IaaGymaaqaaiaadUga % daahaaWcbeqaaiaaiodaaaGccqGHRaWkcaaIXaaaaiabg2da9maala % aabaGaaiikaiaadUgacqGHsislcaaIXaGaaiykaiaacIcacaWGRbWa % aWbaaSqabeaacaaIYaaaaOGaey4kaSIaam4AaiabgUcaRiaaigdaca % GGPaaabaGaaiikaiaadUgacqGHRaWkcaaIXaGaaiykaiaacUfacaGG % OaGaam4AaiabgkHiTiaaigdacaGGPaWaaWbaaSqabeaacaaIYaaaaO % Gaey4kaSIaaiikaiaadUgacqGHsislcaaIXaGaaiykaiabgUcaRiaa % igdacaGGDbaaaaaa!59BD! \frac{{{k^3} - 1}}{{{k^3} + 1}} = \frac{{(k - 1)({k^2} + k + 1)}}{{(k + 1)[{{(k - 1)}^2} + (k - 1) + 1]}}\)
Suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % yDamaaBaaaleaacaWGUbaabeaakiabg2da9maalaaabaGaaGOmaaqa % aiaaiodaaaGaaiOlamaalaaabaGaamOBamaaCaaaleqabaGaaGOmaa % aakiabgUcaRiaad6gacqGHRaWkcaaIXaaabaGaaiikaiaad6gacqGH % sislcaaIXaGaaiykaiaad6gaaaGaeyO0H4TaciiBaiaacMgacaGGTb % GaamyDamaaBaaaleaacaWGUbaabeaakiabg2da9maalaaabaGaaGOm % aaqaaiaaiodaaaaaaa!51E3! \Rightarrow {u_n} = \frac{2}{3}.\frac{{{n^2} + n + 1}}{{(n - 1)n}} \Rightarrow \lim {u_n} = \frac{2}{3}\)