Cho dãy số \(u_n\) với \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaiqaaqaabe % qaaiaadwhadaWgaaWcbaGaaGymaaqabaGccqGH9aqpcaaIXaaabaGa % amyDamaaBaaaleaacaWGUbGaey4kaSIaaGymaaqabaGccqGH9aqpca % WG1bWaaSbaaSqaaiaad6gaaeqaaOGaey4kaSIaamOBamaaCaaaleqa % baGaaGOmaaaaaaGccaGL7baaaaa!4472! \left\{ \begin{array}{l} {u_1} = 1\\ {u_{n + 1}} = {u_n} + {n^2} \end{array} \right.\). Số hạng tổng quát \(u_n\) của dãy số là số hạng nào dưới đây?
Suy nghĩ và trả lời câu hỏi trước khi xem đáp án
Lời giải:
Báo saiTa có:\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaiqaaqaabe % qaaiaadwhadaWgaaWcbaGaaGymaaqabaGccqGH9aqpcaaIXaaabaGa % amyDamaaBaaaleaacaaIYaaabeaakiabg2da9iaadwhadaWgaaWcba % GaaGymaaqabaGccqGHRaWkcaaIXaWaaWbaaSqabeaacaaIYaaaaaGc % baGaamyDamaaBaaaleaacaaIZaaabeaakiabg2da9iaadwhadaWgaa % WcbaGaaGOmaaqabaGccqGHRaWkcaaIYaWaaWbaaSqabeaacaaIYaaa % aaGcbaGaaiOlaiaac6cacaGGUaaabaGaamyDamaaBaaaleaacaWGUb % aabeaakiabg2da9iaadwhadaWgaaWcbaGaamOBaiabgkHiTiaaigda % aeqaaOGaey4kaSYaaeWaaeaacaWGUbGaeyOeI0IaaGymaaGaayjkai % aawMcaamaaCaaaleqabaGaaGOmaaaaaaGccaGL7baaaaa!58A3! \left\{ \begin{array}{l} {u_1} = 1\\ {u_2} = {u_1} + {1^2}\\ {u_3} = {u_2} + {2^2}\\ ...\\ {u_n} = {u_{n - 1}} + {\left( {n - 1} \right)^2} \end{array} \right.\) . Cộng hai vế ta được
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyDamaaBa % aaleaacaWGUbaabeaakiabg2da9iaaigdacqGHRaWkcaaIXaWaaWba % aSqabeaacaaIYaaaaOGaey4kaSIaaGOmamaaCaaaleqabaGaaGOmaa % aakiabgUcaRiaac6cacaGGUaGaaiOlaiabgUcaRmaabmaabaGaamOB % aiabgkHiTiaaigdaaiaawIcacaGLPaaadaahaaWcbeqaaiaaikdaaa % GccqGH9aqpcaaIXaGaey4kaSYaaSaaaeaacaWGUbWaaeWaaeaacaWG % UbGaeyOeI0IaaGymaaGaayjkaiaawMcaamaabmaabaGaaGOmaiaad6 % gacqGHsislcaaIXaaacaGLOaGaayzkaaaabaGaaGOnaaaaaaa!5554! {u_n} = 1 + {1^2} + {2^2} + ... + {\left( {n - 1} \right)^2} = 1 + \frac{{n\left( {n - 1} \right)\left( {2n - 1} \right)}}{6}\)