Hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiabg2 % da9iaadIhadaahaaWcbeqaaiaaiodaaaGccqGHsislcaaIZaGaamiE % amaaCaaaleqabaGaaGOmaaaakiabgkHiTiaaiMdacaWG4bGaey4kaS % IaaGymaaaa!41CD! y = {x^3} - 3{x^2} - 9x + 1\) đồng biến trên khoảng nào trong những khoảng sau?
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Lời giải:
Báo saiTập xác định:D= R . Đạo hàm: \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmyEayaafa % Gaeyypa0JaaG4maiaadIhadaahaaWcbeqaaiaaikdaaaGccqGHsisl % caaI2aGaamiEaiabgkHiTiaaiMdaaaa!3F0B! y' = 3{x^2} - 6x - 9\)
Xét .\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmyEayaafa % Gaeyypa0JaaGimaiabgkDiElaaiodacaWG4bWaaWbaaSqabeaacaaI % YaaaaOGaeyOeI0IaaGOnaiaadIhacqGHsislcaaI5aGaeyypa0JaaG % imaiabgsDiBpaadeaaeaqabeaacaWG4bGaeyypa0JaaG4maiabgkDi % ElaadMhacqGH9aqpcqGHsislcaaIYaGaaGOnaaqaaiaadIhacqGH9a % qpcqGHsislcaaIXaGaeyO0H4TaamyEaiabg2da9iaaiAdaaaGaay5w % aaaaaa!5996! y' = 0 \Rightarrow 3{x^2} - 6x - 9 = 0 \Leftrightarrow \left[ \begin{array}{l} x = 3 \Rightarrow y = - 26\\ x = - 1 \Rightarrow y = 6 \end{array} \right.\)
Bảng biến thiên:
Hàm số đồng biến trên các khoảng \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaacq % GHsislcqGHEisPcaaMc8Uaai4oaiaaykW7cqGHsislcaaIXaaacaGL % OaGaayzkaaaaaa!3F58! \left( { - \infty \,;\, - 1} \right)\) và \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaaca % aIZaGaaGPaVlaacUdacaaMc8Uaey4kaSIaeyOhIukacaGLOaGaayzk % aaaaaa!3E62! \left( {3\,;\, + \infty } \right)\)
Do đó hàm số đồng biến trên khoảng (4;5)