Cho hàm số f(x), đồ thị hàm số f’(x) như hình vẽ.
Hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4zamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg2da9iaadAgadaqadaqaaiaa % dIhadaahaaWcbeqaaiaaikdaaaaakiaawIcacaGLPaaacqGHsislda % WcaaqaaiaadIhadaahaaWcbeqaaiaaiAdaaaaakeaacaaIZaaaaiab % gUcaRiaadIhadaahaaWcbeqaaiaaisdaaaGccqGHsislcaWG4bWaaW % baaSqabeaacaaIYaaaaaaa!4824! g\left( x \right) = f\left( {{x^2}} \right) - \frac{{{x^6}}}{3} + {x^4} - {x^2}\) đạt cực tiểu tại bao nhiêu điểm?
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Lời giải:
Báo saiHD:
Ta có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabm4zayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyypa0JaaGOmaiaadIha % caGGUaGabmOzayaafaWaaeWaaeaacaWG4bWaaWbaaSqabeaacaaIYa % aaaaGccaGLOaGaayzkaaGaeyOeI0IaaGOmaiaadIhadaahaaWcbeqa % aiaaiwdaaaGccqGHRaWkcaaI0aGaamiEamaaCaaaleqabaGaaG4maa % aakiabgkHiTiaaikdacaWG4bGaai4oaiqadEgagaqbamaabmaabaGa % amiEaaGaayjkaiaawMcaaiabg2da9iaaicdacqGHuhY2daWabaabae % qabaGaaGOmaiaadIhacqGH9aqpcaaIWaaabaGabmOzayaafaWaaeWa % aeaacaWG4bWaaWbaaSqabeaacaaIYaaaaaGccaGLOaGaayzkaaGaey % ypa0JaamiEamaaCaaaleqabaGaaGinaaaakiabgkHiTiaaikdacaWG % 4bWaaWbaaSqabeaacaaIYaaaaOGaey4kaSIaaGymaaaacaGLBbaaaa % a!6497! g'\left( x \right) = 2x.f'\left( {{x^2}} \right) - 2{x^5} + 4{x^3} - 2x;g'\left( x \right) = 0 \Leftrightarrow \left[ \begin{array}{l} 2x = 0\\ f'\left( {{x^2}} \right) = {x^4} - 2{x^2} + 1 \end{array} \right.\)Đặt \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiDaiabg2 % da9iaadIhadaahaaWcbeqaaiaaikdaaaGccqGHLjYScaaIWaaaaa!3C62! t = {x^2} \ge 0\) nên phương trình trở thành:\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmOzayaafa % WaaeWaaeaacaWG0baacaGLOaGaayzkaaGaeyypa0JaamiDamaaCaaa % leqabaGaaGOmaaaakiabgkHiTiaaikdacaWG0bGaey4kaSIaaGymaa % aa!409D! f'\left( t \right) = {t^2} - 2t + 1\)
Dựa vào hình vẽ, ta thấy (*) có hai nghiệm phân biệt \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiDaiabg2 % da9iaaigdacaGG7aGaamiDaiabg2da9iaaikdacqGHshI3daWabaab % aeqabaGaamiEaiabg2da9iabgglaXkaaigdaaeaacaWG4bGaeyypa0 % JaeyySae7aaOaaaeaacaaIYaaaleqaaaaakiaawUfaaaaa!4904! t = 1;t = 2 \Rightarrow \left[ \begin{array}{l} x = \pm 1\\ x = \pm \sqrt 2 \end{array} \right.\).
Lập bảng biến thiên ta thấy Hàm số y = g(x) có một điểm cực tiểu
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
Tuyển chọn số 5