Cho hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg2da9iGacYgacaGGVbGaai4z % amaaBaaaleaadaWcaaqaaiaaigdaaeaacaaIZaaaaaqabaGcdaqada % qaaiaadIhadaahaaWcbeqaaiaaikdaaaGccqGHsislcaaI1aGaamiE % aiabgUcaRiaaiEdaaiaawIcacaGLPaaaaaa!46BE! f\left( x \right) = {\log _{\frac{1}{3}}}\left( {{x^2} - 5x + 7} \right)\). Nghiệm của bất phương trình f(x) > 0 là:
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Lời giải:
Báo saiĐiều kiện , \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEamaaCa % aaleqabaGaaGOmaaaakiabgkHiTiaaiwdacaWG4bGaey4kaSIaaG4n % aiabg6da+iaaicdacaGGSaGaeyiaIiIaamiEaaaa!406E! {x^2} - 5x + 7 > 0,\forall x\)
Ta có : \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg6da+iaaicdacqGHuhY2ciGG % SbGaai4BaiaacEgadaWgaaWcbaWaaSaaaeaacaaIXaaabaGaaG4maa % aaaeqaaOWaaeWaaeaacaWG4bWaaWbaaSqabeaacaaIYaaaaOGaeyOe % I0IaaGynaiaadIhacqGHRaWkcaaI3aaacaGLOaGaayzkaaGaeyOpa4 % JaaGimaiabgsDiBlaadIhadaahaaWcbeqaaiaaikdaaaGccqGHsisl % caaI1aGaamiEaiabgUcaRiaaiEdacqGH8aapcaaIXaGaeyi1HSTaam % iEamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaaiwdacaWG4bGaey4k % aSIaaGOnaiabgYda8iaaicdacqGHuhY2caaIYaGaeyipaWJaamiEai % abgYda8iaaiodaaaa!671E! f\left( x \right) > 0 \Leftrightarrow {\log _{\frac{1}{3}}}\left( {{x^2} - 5x + 7} \right) > 0 \Leftrightarrow {x^2} - 5x + 7 < 1 \Leftrightarrow {x^2} - 5x + 6 < 0 \Leftrightarrow 2 < x < 3\)Kết hợp với điều kiện \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEaiabgw % MiZkaaigdaaaa!3972! x \ge 1\)