Mỗi chuyến xe buýt có sức chứa tối đa là 60 hành khách. Một chuyến xe buýt chở x hành khách thì giá tiền cho mỗi hành khách là \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVCI8FfYJH8YrFfeuY-Hhbbf9v8qqaqFr0xc9pk0xbb % a9q8WqFfeaY-biLkVcLq-JHqpepeea0-as0Fb9pgeaYRXxe9vr0-vr % 0-vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaamaabmaabaGaaG % 4maiabgkHiTmaalaaabaGaamiEaaqaaiaaisdacaaIWaaaaaGaayjk % aiaawMcaamaaCaaaleqabaGaaGOmaaaaaaa!3C8A! {\left( {3 - \frac{x}{{40}}} \right)^2}(USD)\) . Khẳng định nào sau đây đúng.
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Lời giải:
Báo saiSố tiền thu được là: \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyEaiabg2 % da9iaadIhadaqadaqaaiaaiodacqGHsisldaWcaaqaaiaadIhaaeaa % caaI0aGaaGimaaaaaiaawIcacaGLPaaadaahaaWcbeqaaiaaikdaaa % GccqGHshI3ceWG5bGbauaacqGH9aqpcaaI5aGaeyOeI0YaaSaaaeaa % caaIZaaabaGaaGymaiaaicdaaaGaamiEaiabgUcaRmaalaaabaGaaG % 4maiaadIhadaahaaWcbeqaaiaaikdaaaaakeaacaaIXaGaaGOnaiaa % icdacaaIWaaaaiabg2da9iaaicdacqGHuhY2daWabaabaeqabaGaam % iEaiabg2da9iaaisdacaaIWaaabaGaamiEaiabg2da9iaaigdacaaI % YaGaaGimaaaacaGLBbaacaqGGaGaaeiiaiaaicdacqGHKjYOcaWG4b % GaeyizImQaaGOnaiaaicdaaaa!6438! y = x{\left( {3 - \frac{x}{{40}}} \right)^2} \Rightarrow y' = 9 - \frac{3}{{10}}x + \frac{{3{x^2}}}{{1600}} = 0 \Leftrightarrow \left[ \begin{array}{l} x = 40\\ x = 120 \end{array} \right.{\rm{ }}0 \le x \le 60\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % yEamaaBaaaleaaciGGTbGaaiyyaiaacIhaaeqaaOGaeyypa0JaaGym % aiaaiAdacaaIWaGaeyi1HSTaamiEaiabg2da9iaaisdacaaIWaaaaa!456B! \Rightarrow {y_{\max }} = 160 \Leftrightarrow x = 40\)