Cho hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiabg2 % da9maalaaabaWaaOaaaeaacaaIYaaaleqaaaGcbaGaci4yaiaac+ga % caGGZbGaaG4maiaadIhaaaaaaa!3D76! y = \frac{{\sqrt 2 }}{{\cos 3x}}\). Khi đó \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmyEayaafa % WaaeWaaeaadaWcaaqaaiabec8aWbqaaiaaiodaaaaacaGLOaGaayzk % aaaaaa!3B11! y'\left( {\frac{\pi }{3}} \right)\) là:
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Lời giải:
Báo sai\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmyEayaafa % Gaeyypa0JaeyOeI0YaaOaaaeaacaaIYaaaleqaaOGaaiOlamaalaaa % baWaaeWaaeaaciGGJbGaai4BaiaacohacaaIZaGaamiEaaGaayjkai % aawMcaamaaCaaaleqabaGccWaGGBOmGikaaaqaaiGacogacaGGVbGa % ai4CamaaCaaaleqabaGaaGOmaaaakiaaiodacaWG4baaaiabg2da9m % aalaaabaGaaG4mamaakaaabaGaaGOmaaWcbeaakiaac6caciGGZbGa % aiyAaiaac6gacaaIZaGaamiEaaqaaiGacogacaGGVbGaai4CamaaCa % aaleqabaGaaGOmaaaakiaaiodacaWG4baaaaaa!56C0! y' = - \sqrt 2 .\frac{{{{\left( {\cos 3x} \right)}^\prime }}}{{{{\cos }^2}3x}} = \frac{{3\sqrt 2 .\sin 3x}}{{{{\cos }^2}3x}}\)
do đó \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiaacE % cadaqadaqaamaalaaabaGaeqiWdahabaGaaG4maaaaaiaawIcacaGL % PaaacqGH9aqpdaWcaaqaaiaaiodadaGcaaqaaiaaikdaaSqabaGcca % GGUaGaci4CaiaacMgacaGGUbGaeqiWdahabaGaci4yaiaac+gacaGG % ZbWaaWbaaSqabeaacaaIYaaaaOGaeqiWdahaaiabg2da9iaaicdaaa % a!4AEE! y'\left( {\frac{\pi }{3}} \right) = \frac{{3\sqrt 2 .\sin \pi }}{{{{\cos }^2}\pi }} = 0\)