Hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiabg2 % da9maabmaabaGaamiEamaaCaaaleqabaGaaG4maaaakiabgkHiTiaa % iodacaWG4baacaGLOaGaayzkaaWaaWbaaSqabeaacaWGLbaaaaaa!3F30! y = {\left( {{x^3} - 3x} \right)^e}\) có bao nhiêu điểm cực trị?
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Lời giải:
Báo saiĐiều kiện: \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEamaaCa % aaleqabaGaaG4maaaakiabgkHiTiaaiodacaWG4bGaeyOpa4JaaGim % aiabgsDiBlaadIhadaqadaqaaiaadIhadaahaaWcbeqaaiaaikdaaa % GccqGHsislcaaIZaaacaGLOaGaayzkaaGaeyOpa4JaaGimaiabgsDi % BlaadIhadaqadaqaaiaadIhacqGHsisldaGcaaqaaiaaiodaaSqaba % aakiaawIcacaGLPaaadaqadaqaaiaadIhacqGHRaWkdaGcaaqaaiaa % iodaaSqabaaakiaawIcacaGLPaaacqGH+aGpcaaIWaGaeyi1HS9aam % qaaqaabeqaaiabgkHiTmaakaaabaGaaG4maaWcbeaakiabgYda8iaa % dIhacqGH8aapcaaIWaaabaGaamiEaiabg6da+maakaaabaGaaG4maa % WcbeaaaaGccaGLBbaaaaa!6019! {x^3} - 3x > 0 \Leftrightarrow x\left( {{x^2} - 3} \right) > 0 \Leftrightarrow x\left( {x - \sqrt 3 } \right)\left( {x + \sqrt 3 } \right) > 0 \Leftrightarrow \left[ \begin{array}{l} - \sqrt 3 < x < 0\\ x > \sqrt 3 \end{array} \right.\)
Ta có: \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiaacE % cacqGH9aqpcaWGLbWaaeWaaeaacaaIZaGaamiEamaaCaaaleqabaGa % aGOmaaaakiabgkHiTiaaiodaaiaawIcacaGLPaaadaqadaqaaiaadI % hadaahaaWcbeqaaiaaiodaaaGccqGHsislcaaIZaGaamiEaaGaayjk % aiaawMcaamaaCaaaleqabaGaamyzaiabgkHiTiaaigdaaaaaaa!484D! y' = e\left( {3{x^2} - 3} \right){\left( {{x^3} - 3x} \right)^{e - 1}}\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % yEaiaacEcacqGH9aqpcaaIWaGaeyi1HS9aaeWaaeaacaaIZaGaamiE % amaaCaaaleqabaGaaGOmaaaakiabgkHiTiaaiodaaiaawIcacaGLPa % aadaqadaqaaiaadIhadaahaaWcbeqaaiaaiodaaaGccqGHsislcaaI % ZaGaamiEaaGaayjkaiaawMcaamaaCaaaleqabaGaamyzaiabgkHiTi % aaigdaaaGccqGH9aqpcaaIWaGaeyi1HSTaaG4maiaadIhadaahaaWc % beqaaiaaikdaaaGccqGHsislcaaIZaGaeyypa0JaaGimaiabgsDiBp % aadeaaeaqabeaacaWG4bGaeyypa0JaeyOeI0IaaGymaaqaaiaadIha % cqGH9aqpcaaIXaaaaiaawUfaaaaa!60DA! \Rightarrow y' = 0 \Leftrightarrow \left( {3{x^2} - 3} \right){\left( {{x^3} - 3x} \right)^{e - 1}} = 0 \Leftrightarrow 3{x^2} - 3 = 0 \Leftrightarrow \left[ \begin{array}{l} x = - 1\\ x = 1 \end{array} \right.\)
Ta có bảng xét dấu:
Dựa vào bảng xét dấu của hàm số ta thấy đạo hàm của hàm số chỉ đổi dấu qua 1 điểm \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEaiabg2 % da9iabgkHiTiaaigdacqGHshI3aaa!3BFC! x = - 1 \Rightarrow \) hàm số có 1 điểm cực trị.
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
Tuyển chọn số 4