Tính giới hạn của dãy số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyDamaaBa % aaleaacaWGUbaabeaakiabg2da9maalaaabaGaaGymaaqaaiaaikda % daGcaaqaaiaaigdaaSqabaGccqGHRaWkdaGcaaqaaiaaikdaaSqaba % aaaOGaey4kaSYaaSaaaeaacaaIXaaabaGaaG4mamaakaaabaGaaGOm % aaWcbeaakiabgUcaRiaaikdadaGcaaqaaiaaiodaaSqabaaaaOGaey % 4kaSIaaiOlaiaac6cacaGGUaGaey4kaSYaaSaaaeaacaaIXaaabaGa % aiikaiaad6gacqGHRaWkcaaIXaGaaiykamaakaaabaGaamOBaaWcbe % aakiabgUcaRiaad6gadaGcaaqaaiaad6gacqGHRaWkcaaIXaaaleqa % aaaaaaa!5138! {u_n} = \frac{1}{{2\sqrt 1 + \sqrt 2 }} + \frac{1}{{3\sqrt 2 + 2\sqrt 3 }} + ... + \frac{1}{{(n + 1)\sqrt n + n\sqrt {n + 1} }}\)
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Lời giải:
Báo saiTa có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % aIXaaabaGaaiikaiaadUgacqGHRaWkcaaIXaGaaiykamaakaaabaGa % am4AaaWcbeaakiabgUcaRiaadUgadaGcaaqaaiaadUgacqGHRaWkca % aIXaaaleqaaaaakiabg2da9maalaaabaGaaGymaaqaamaakaaabaGa % am4AaaWcbeaaaaGccqGHsisldaWcaaqaaiaaigdaaeaadaGcaaqaai % aadUgacqGHRaWkcaaIXaaaleqaaaaaaaa!4784! \frac{1}{{(k + 1)\sqrt k + k\sqrt {k + 1} }} = \frac{1}{{\sqrt k }} - \frac{1}{{\sqrt {k + 1} }}\)
Suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyDamaaBa % aaleaacaWGUbaabeaakiabg2da9iaaigdacqGHsisldaWcaaqaaiaa % igdaaeaadaGcaaqaaiaad6gacqGHRaWkcaaIXaaaleqaaaaakiabgk % DiElGacYgacaGGPbGaaiyBaiaadwhadaWgaaWcbaGaamOBaaqabaGc % cqGH9aqpcaaIXaaaaa!4756! {u_n} = 1 - \frac{1}{{\sqrt {n + 1} }} \Rightarrow \lim {u_n} = 1\)