Cho hàm số y = f(x) có đạo hàm \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmOzayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyypa0ZaaeWaaeaacaWG % 4bGaeyOeI0IaaGymaaGaayjkaiaawMcaamaaCaaaleqabaGaaGOmaa % aakmaabmaabaGaamiEamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaa % ikdacaWG4baacaGLOaGaayzkaaaaaa!45B6! f'\left( x \right) = {\left( {x - 1} \right)^2}\left( {{x^2} - 2x} \right)\) với \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyiaIiIaam % iEaiabgIGiolabl2riHcaa!3AB4! \forall x \in R\). Có bao nhiêu giá trị nguyên dương của tham số m để hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzamaabm % aabaGaamiEamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaaiIdacaWG % 4bGaey4kaSIaamyBaaGaayjkaiaawMcaaaaa!3ED7! f\left( {{x^2} - 8x + m} \right)\) có 5 điểm cực trị?
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Lời giải:
Báo saiĐặt \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4zamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg2da9iaadAgadaqadaqaaiaa % dIhadaahaaWcbeqaaiaaikdaaaGccqGHsislcaaI4aGaamiEaiabgU % caRiaad2gaaiaawIcacaGLPaaaaaa!434F! g\left( x \right) = f\left( {{x^2} - 8x + m} \right)\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmOzayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyypa0ZaaeWaaeaacaWG % 4bGaeyOeI0IaaGymaaGaayjkaiaawMcaamaaCaaaleqabaGaaGOmaa % aakmaabmaabaGaamiEamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaa % ikdacaWG4baacaGLOaGaayzkaaGaeyO0H4naaa!4813! f'\left( x \right) = {\left( {x - 1} \right)^2}\left( {{x^2} - 2x} \right) \Rightarrow \)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabm4zayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyypa0ZaaeWaaeaacaaI % YaGaamiEaiabgkHiTiaaiIdaaiaawIcacaGLPaaadaqadaqaaiaadI % hadaahaaWcbeqaaiaaikdaaaGccqGHsislcaaI4aGaamiEaiabgUca % Riaad2gacqGHsislcaaIXaaacaGLOaGaayzkaaWaaWbaaSqabeaaca % aIYaaaaOWaaeWaaeaacaWG4bWaaWbaaSqabeaacaaIYaaaaOGaeyOe % I0IaaGioaiaadIhacqGHRaWkcaWGTbaacaGLOaGaayzkaaWaaeWaae % aacaWG4bWaaWbaaSqabeaacaaIYaaaaOGaeyOeI0IaaGioaiaadIha % cqGHRaWkcaWGTbGaeyOeI0IaaGOmaaGaayjkaiaawMcaaaaa!5B97! g'\left( x \right) = \left( {2x - 8} \right){\left( {{x^2} - 8x + m - 1} \right)^2}\left( {{x^2} - 8x + m} \right)\left( {{x^2} - 8x + m - 2} \right)\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabm4zayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyypa0JaaGimaaaa!3B31! g'\left( x \right) = 0\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HS9aam % qaaqaabeqaaiaadIhacqGH9aqpcaaI0aaabaGaamiEamaaCaaaleqa % baGaaGOmaaaakiabgkHiTiaaiIdacaWG4bGaey4kaSIaamyBaiabgk % HiTiaaigdacqGH9aqpcaaIWaGaaGPaVlaaykW7caaMc8UaaGPaVlaa % ykW7daqadaqaaiaaigdaaiaawIcacaGLPaaaaeaacaWG4bWaaWbaaS % qabeaacaaIYaaaaOGaeyOeI0IaaGioaiaadIhacqGHRaWkcaWGTbGa % eyypa0JaaGimaiaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVl % aaykW7caaMc8UaaGPaVlaaykW7caaMc8+aaeWaaeaacaaIYaaacaGL % OaGaayzkaaaabaGaamiEamaaCaaaleqabaGaaGOmaaaakiabgkHiTi % aaiIdacaWG4bGaey4kaSIaamyBaiabgkHiTiaaikdacqGH9aqpcaaI % WaGaaGPaVlaaykW7caaMc8+aaeWaaeaacaaIZaaacaGLOaGaayzkaa % aaaiaawUfaaaaa!7C15! \Leftrightarrow \left[ \begin{array}{l} x = 4\\ {x^2} - 8x + m - 1 = 0\,\,\,\,\,\left( 1 \right)\\ {x^2} - 8x + m = 0\,\,\,\,\,\,\,\,\,\,\,\left( 2 \right)\\ {x^2} - 8x + m - 2 = 0\,\,\,\left( 3 \right) \end{array} \right.\)
Các phương trình (1),(2) ,(3) không có nghiệm chung từng đôi một và \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaaca % WG4bWaaWbaaSqabeaacaaIYaaaaOGaeyOeI0IaaGioaiaadIhacqGH % RaWkcaWGTbGaeyOeI0IaaGymaaGaayjkaiaawMcaamaaCaaaleqaba % GaaGOmaaaakiabgwMiZkaaicdaaaa!4307! {\left( {{x^2} - 8x + m - 1} \right)^2} \ge 0\) với \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyiaIiIaam % iEaiabgIGiolabl2riHcaa!3AB4! \forall x \in R\)
Suy ra g(x) có 5 điểm cực trị khi và chỉ khi (2) và (3) có hai nghiệm phân biệt khác 4.
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HS9aai % qaaqaabeqaaiabfs5aenaaBaaaleaacaaIYaaabeaakiabg2da9iaa % igdacaaI2aGaeyOeI0IaamyBaiabg6da+iaaicdaaeaacqqHuoarda % WgaaWcbaGaaG4maaqabaGccqGH9aqpcaaIXaGaaGOnaiabgkHiTiaa % d2gacqGHRaWkcaaIYaGaeyOpa4JaaGimaaqaaiaaigdacaaI2aGaey % OeI0IaaG4maiaaikdacqGHRaWkcaWGTbGaeyiyIKRaaGimaaqaaiaa % igdacaaI2aGaeyOeI0IaaG4maiaaikdacqGHRaWkcaWGTbGaeyOeI0 % IaaGOmaiabgcMi5kaaicdaaaGaay5Eaaaaaa!5E1A! \Leftrightarrow \left\{ \begin{array}{l} {\Delta _2} = 16 - m > 0\\ {\Delta _3} = 16 - m + 2 > 0\\ 16 - 32 + m \ne 0\\ 16 - 32 + m - 2 \ne 0 \end{array} \right.\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HS9aai % qaaqaabeqaaiaad2gacqGH8aapcaaIXaGaaGOnaaqaaiaad2gacqGH % 8aapcaaIXaGaaGioaaqaaiaad2gacqGHGjsUcaaIXaGaaGOnaaqaai % aad2gacqGHGjsUcaaIXaGaaGioaaaacaGL7baaaaa!48C0! \Leftrightarrow \left\{ \begin{array}{l} m < 16\\ m < 18\\ m \ne 16\\ m \ne 18 \end{array} \right.\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HSTaam % yBaiabgYda8iaaigdacaaI2aaaaa!3BC0! \Leftrightarrow m < 16\)
Vì m nguyên dương và m < 16 nên có 15 giá trị m cần tìm.
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
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