Cho y =f(x) mà đồ thị hàm số y =f'(x) như hình bên. Hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiabg2 % da9iaadAgadaqadaqaaiaadIhacqGHsislcaaIXaaacaGLOaGaayzk % aaGaey4kaSIaamiEamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaaik % dacaWG4baaaa!4289! y = f\left( {x - 1} \right) + {x^2} - 2x\) đồng biến trên khoảng?
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Lời giải:
Báo saiTa có: \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiaacE % cacqGH9aqpcaWGMbGaai4jamaabmaabaGaamiEaiabgkHiTiaaigda % aiaawIcacaGLPaaacqGHRaWkcaaIYaGaamiEaiabgkHiTiaaikdacq % GH9aqpcaaIWaGaeyi1HSTaamOzaiaacEcadaqadaqaaiaadIhacqGH % sislcaaIXaaacaGLOaGaayzkaaGaey4kaSIaaGOmamaabmaabaGaam % iEaiabgkHiTiaaigdaaiaawIcacaGLPaaacqGH9aqpcaaIWaaaaa!5417! y' = f'\left( {x - 1} \right) + 2x - 2 = 0 \Leftrightarrow f'\left( {x - 1} \right) + 2\left( {x - 1} \right) = 0\)
Đặt t = x - 1 ta có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiaacE % cacaWGMbGaai4jamaabmaabaGaamiDaaGaayjkaiaawMcaaiabgUca % RiaaikdacaWG0bGaeyypa0JaaGimaiabgsDiBlaadAgacaGGNaWaae % WaaeaacaWG0baacaGLOaGaayzkaaGaeyOeI0YaaeWaaeaacqGHsisl % caaIYaGaamiDaaGaayjkaiaawMcaaiabg2da9iaaicdaaaa!4D58! y'f'\left( t \right) + 2t = 0 \Leftrightarrow f'\left( t \right) - \left( { - 2t} \right) = 0\)
Vẽ đồ thị hàm số y = f'(t) và y = -2t trên cùng mặt phẳng tọa độ ta có:
Xét \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiaacE % cacqGHLjYScaaIWaGaeyi1HSTaamOzaiaacEcadaqadaqaaiaadsha % aiaawIcacaGLPaaacqGHLjYScqGHsislcaaIYaGaamiDaiabgkDiEd % aa!4756! y' \ge 0 \Leftrightarrow f'\left( t \right) \ge - 2t \Rightarrow \) Đồ thị hàm số y = f'(t) nằm trên đường thẳng y = -2t
Xét \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEaiabgI % GiopaabmaabaGaaGymaiaacUdacaaIYaaacaGLOaGaayzkaaGaeyO0 % H4TaamiDaiabgIGiopaabmaabaGaaGimaiaacUdacaaIXaaacaGLOa % GaayzkaaGaeyO0H4naaa!4728! x \in \left( {1;2} \right) \Rightarrow t \in \left( {0;1} \right) \Rightarrow \) thỏa mãn.
Xét \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEaiabgI % GiopaabmaabaGaeyOeI0IaaGymaiaacUdacaaIWaaacaGLOaGaayzk % aaGaeyO0H4TaamiDaiabgIGiopaabmaabaGaeyOeI0IaaGOmaiaacU % dacqGHsislcaaIXaaacaGLOaGaayzkaaGaeyO0H4naaa!49EF! x \in \left( { - 1;0} \right) \Rightarrow t \in \left( { - 2; - 1} \right) \Rightarrow \) không thỏa mãn.
Xét \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEaiabgI % GiopaabmaabaGaaGimaiaacUdacaaIXaaacaGLOaGaayzkaaGaeyO0 % H4TaamiDaiabgIGiopaabmaabaGaeyOeI0IaaGymaiaacUdacaaIWa % aacaGLOaGaayzkaaGaeyO0H4naaa!4813! x \in \left( {0;1} \right) \Rightarrow t \in \left( { - 1;0} \right) \Rightarrow \) không thỏa mãn.
Xét \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEaiabgI % GiopaabmaabaGaeyOeI0IaaGOmaiaacUdacqGHsislcaaIXaaacaGL % OaGaayzkaaGaeyO0H4TaamiDaiabgIGiopaabmaabaGaeyOeI0IaaG % 4maiaacUdacqGHsislcaaIYaaacaGLOaGaayzkaaGaeyO0H4naaa!4AE0! x \in \left( { - 2; - 1} \right) \Rightarrow t \in \left( { - 3; - 2} \right) \Rightarrow \) không thỏa mãn.
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
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