Tính giới hạn của dãy số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyDamaaBa % aaleaacaWGUbaabeaakiabg2da9maaqahabaWaaSaaaeaacaWGUbaa % baGaamOBamaaCaaaleqabaGaaGOmaaaakiabgUcaRiaadUgaaaaale % aacaWGRbGaeyypa0JaaGymaaqaaiaad6gaa0GaeyyeIuoaaaa!43BE! {u_n} = \sum\limits_{k = 1}^n {\frac{n}{{{n^2} + k}}} \)
Hãy suy nghĩ và trả lời câu hỏi trước khi xem đáp án
Lời giải:
Báo saiTa có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOBamaala % aabaGaamOBaaqaaiaad6gadaahaaWcbeqaaiaaikdaaaGccqGHRaWk % caWGUbaaaiabgsMiJkaadwhadaWgaaWcbaGaamOBaaqabaGccqGHKj % YOcaWGUbWaaSaaaeaacaWGUbaabaGaamOBamaaCaaaleqabaGaaGOm % aaaakiabgUcaRiaaigdaaaGaeyO0H49aaSaaaeaacqGHsislcaWGUb % aabaGaamOBamaaCaaaleqabaGaaGOmaaaakiabgUcaRiaaigdaaaGa % eyizImQaamyDamaaBaaaleaacaWGUbaabeaakiabgkHiTiaaigdacq % GHKjYOdaWcaaqaaiabgkHiTiaaigdaaeaacaWGUbWaaWbaaSqabeaa % caaIYaaaaOGaey4kaSIaaGymaaaaaaa!5AEB! n\frac{n}{{{n^2} + n}} \le {u_n} \le n\frac{n}{{{n^2} + 1}} \Rightarrow \frac{{ - n}}{{{n^2} + 1}} \le {u_n} - 1 \le \frac{{ - 1}}{{{n^2} + 1}}\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H49aaq % WaaeaacaWG1bWaaSbaaSqaaiaad6gaaeqaaOGaeyOeI0IaaGymaaGa % ay5bSlaawIa7aiabgsMiJoaalaaabaGaamOBaaqaaiaad6gadaahaa % WcbeqaaiaaikdaaaGccqGHRaWkcaaIXaaaaiabgkziUkaaicdacqGH % shI3ciGGSbGaaiyAaiaac2gacaWG1bWaaSbaaSqaaiaad6gaaeqaaO % Gaeyypa0JaaGymaaaa!5131! \Rightarrow \left| {{u_n} - 1} \right| \le \frac{n}{{{n^2} + 1}} \to 0 \Rightarrow \lim {u_n} = 1\)