A. Nếu \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGacaGaaiaabeqaamaabaabaaGcbaGabmOzayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyyzImRaaGimaiaaykW7 % caaMc8UaeyiaIiIaamiEaiabgIGiopaabmaabaGaamyyaiaacUdaca % WGIbaacaGLOaGaayzkaaaaaa!466B! f'\left( x \right) \ge 0\,\,\forall x \in \left( {a;b} \right)\) thì hàm số y = f(x) đồng biến trên (a;b).
B. Nếu \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGacaGaaiaabeqaamaabaabaaGcbaGabmOzayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyOpa4JaaGimaiaaykW7 % caaMc8UaeyiaIiIaamiEaiabgIGiopaabmaabaGaamyyaiaacUdaca % WGIbaacaGLOaGaayzkaaaaaa!45AD! f'\left( x \right) > 0\,\,\forall x \in \left( {a;b} \right)\) thì hàm số y = f(x) đồng biến trên (a,b).
C. Hàm số y = f(x) đồng biến trên (a,b) khi và chỉ khi \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGacaGaaiaabeqaamaabaabaaGcbaGabmOzayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyyzImRaaGimaiaaykW7 % caaMc8UaeyiaIiIaamiEaiabgIGiopaabmaabaGaamyyaiaacUdaca % WGIbaacaGLOaGaayzkaaaaaa!466B! f'\left( x \right) \ge 0\,\,\forall x \in \left( {a;b} \right)\) .
D. Hàm số y = f(x) đồng biến trên (a,b) khi và chỉ khi \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGacaGaaiaabeqaamaabaabaaGcbaGabmOzayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyOpa4JaaGimaiaaykW7 % caaMc8UaeyiaIiIaamiEaiabgIGiopaabmaabaGaamyyaiaacUdaca % WGIbaacaGLOaGaayzkaaaaaa!45AD! f'\left( x \right) > 0\,\,\forall x \in \left( {a;b} \right)\)
Ta có hàm số y = f(x) đồng biến trên (a;b) khi và chỉ khi \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGacaGaaiaabeqaamaabaabaaGcbaGabmOzayaafa % WaaeWaaeaacaWG4baacaGLOaGaayzkaaGaeyyzImRaaGimaiaaykW7 % caaMc8UaeyiaIiIaamiEaiabgIGiopaabmaabaGaamyyaiaacUdaca % WGIbaacaGLOaGaayzkaaaaaa!466B! f'\left( x \right) \ge 0\,\,\forall x \in \left( {a;b} \right)\) , trong đó f(x) = 0 tại hữu hạn điểm thuộc (a;b) Do đó phương án A, C, D sai.