Tính giới hạn của dãy số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyDamaaBa % aaleaacaWGUbaabeaakiabg2da9iaadghacqGHRaWkcaaIYaGaamyC % amaaCaaaleqabaGaaGOmaaaakiabgUcaRiaac6cacaGGUaGaaiOlai % abgUcaRiaad6gacaWGXbWaaWbaaSqabeaacaWGUbaaaaaa!447C! {u_n} = q + 2{q^2} + ... + n{q^n}\) với |q| < 1
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Lời giải:
Báo saiTa có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyDamaaBa % aaleaacaWGUbaabeaakiabgkHiTiaadghacaWG1bWaaSbaaSqaaiaa % d6gaaeqaaOGaeyypa0JaamyCaiabgUcaRiaadghadaahaaWcbeqaai % aaikdaaaGccqGHRaWkcaWGXbWaaWbaaSqabeaacaaIZaaaaOGaey4k % aSIaaiOlaiaac6cacaGGUaGaey4kaSIaamyCamaaCaaaleqabaGaam % OBaaaakiabgkHiTiaad6gacaWGXbWaaWbaaSqabeaacaWGUbGaey4k % aSIaaGymaaaaaaa!4F3D! {u_n} - q{u_n} = q + {q^2} + {q^3} + ... + {q^n} - n{q^{n + 1}}\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taai % ikaiaaigdacqGHsislcaWGXbGaaiykaiaadwhadaWgaaWcbaGaamOB % aaqabaGccqGH9aqpcaWGXbWaaSaaaeaacaaIXaGaeyOeI0IaamyCam % aaCaaaleqabaGaamOBaaaaaOqaaiaaigdacqGHsislcaWGXbaaaiab % gkHiTiaad6gacaWGXbWaaWbaaSqabeaacaWGUbGaey4kaSIaaGymaa % aaaaa!4C70! \Rightarrow (1 - q){u_n} = q\frac{{1 - {q^n}}}{{1 - q}} - n{q^{n + 1}}\). Suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaacM % gacaGGTbGaamyDamaaBaaaleaacaWGUbaabeaakiabg2da9maalaaa % baGaamyCaaqaamaabmaabaGaaGymaiabgkHiTiaadghaaiaawIcaca % GLPaaadaahaaWcbeqaaiaaikdaaaaaaaaa!4203! \lim {u_n} = \frac{q}{{{{\left( {1 - q} \right)}^2}}}\)