Hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg2da9maaemaabaWaaSaaaeaa % caWG4baabaGaamiEamaaCaaaleqabaGaaGOmaaaakiabgUcaRiaaig % daaaGaeyOeI0IaamyBaaGaay5bSlaawIa7aaaa!4406! f\left( x \right) = \left| {\frac{x}{{{x^2} + 1}} - m} \right|\) (với m là tham số thực) có nhiều nhất bao nhiêu điểm cực trị?
Hãy suy nghĩ và trả lời câu hỏi trước khi xem đáp án
Lời giải:
Báo saiHàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg2da9maaemaabaWaaSaaaeaa % caWG4baabaGaamiEamaaCaaaleqabaGaaGOmaaaakiabgUcaRiaaig % daaaGaeyOeI0IaamyBaaGaay5bSlaawIa7aaaa!4406! f\left( x \right) = \left| {\frac{x}{{{x^2} + 1}} - m} \right|\) có TXĐ: D = R
Xét hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4zamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg2da9maalaaabaGaamiEaaqa % aiaadIhadaahaaWcbeqaaiaaikdaaaGccqGHRaWkcaaIXaaaaiabgk % HiTiaad2gaaaa!40E5! g\left( x \right) = \frac{x}{{{x^2} + 1}} - m\) ta có:
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4zaiaacE % cadaqadaqaaiaadIhaaiaawIcacaGLPaaacqGH9aqpdaWcaaqaaiaa % dIhadaahaaWcbeqaaiaaikdaaaGccqGHRaWkcaaIXaGaeyOeI0Iaam % iEaiaac6cacaaIYaGaamiEaaqaamaabmaabaGaamiEamaaCaaaleqa % baGaaGOmaaaakiabgUcaRiaaigdaaiaawIcacaGLPaaadaahaaWcbe % qaaiaaikdaaaaaaOGaeyypa0ZaaSaaaeaacqGHsislcaWG4bWaaWba % aSqabeaacaaIYaaaaOGaey4kaSIaaGymaaqaamaabmaabaGaamiEam % aaCaaaleqabaGaaGOmaaaakiabgUcaRiaaigdaaiaawIcacaGLPaaa % daahaaWcbeqaaiaaikdaaaaaaOGaeyypa0JaaGimaiabgsDiBlaadI % hacqGH9aqpcqGHXcqScaaIXaaaaa!5D73! g'\left( x \right) = \frac{{{x^2} + 1 - x.2x}}{{{{\left( {{x^2} + 1} \right)}^2}}} = \frac{{ - {x^2} + 1}}{{{{\left( {{x^2} + 1} \right)}^2}}} = 0 \Leftrightarrow x = \pm 1\)
Hàm số y = g(x) có 2 điểm cực trị.
Xét phương trình hoành độ giao điểm \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % WG4baabaGaamiEamaaCaaaleqabaGaaGOmaaaakiabgUcaRiaaigda % aaGaeyOeI0IaamyBaiabg2da9iaaicdacqGHuhY2daWcaaqaaiaadI % hacqGHsislcaWGTbWaaeWaaeaacaWG4bWaaWbaaSqabeaacaaIYaaa % aOGaey4kaSIaaGymaaGaayjkaiaawMcaaaqaaiaadIhadaahaaWcbe % qaaiaaikdaaaGccqGHRaWkcaaIXaaaaiabg2da9iaaicdacqGHuhY2 % cqGHsislcaWGTbGaamiEamaaCaaaleqabaGaaGOmaaaakiabgUcaRi % aadIhacqGHsislcaWGTbGaeyypa0JaaGimaaaa!5981! \frac{x}{{{x^2} + 1}} - m = 0 \Leftrightarrow \frac{{x - m\left( {{x^2} + 1} \right)}}{{{x^2} + 1}} = 0 \Leftrightarrow - m{x^2} + x - m = 0\), phương trình có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeuiLdqKaey % ypa0JaaGymaiabgkHiTiaaisdacaWGTbWaaWbaaSqabeaacaaIYaaa % aaaa!3CA1! \Delta = 1 - 4{m^2}\) chưa xác định dấu nên có tối đa 2 nghiệm.
Vậy hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg2da9maaemaabaWaaSaaaeaa % caWG4baabaGaamiEamaaCaaaleqabaGaaGOmaaaakiabgUcaRiaaig % daaaGaeyOeI0IaamyBaaGaay5bSlaawIa7aaaa!4406! f\left( x \right) = \left| {\frac{x}{{{x^2} + 1}} - m} \right|\) có tối đa 2 + 2 = 4 cực trị.
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
Tuyển chọn số 4