Trung Tâm Hiếu Học Minh Châu của Thầy Trần chứa tối đa mỗi phòng học là 200 em HS. Nếu một phòng học có x HS thì học phí cho mỗi HS là \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaaca % aI5aGaeyOeI0YaaSaaaeaacaWG4baabaGaaGinaiaaicdaaaaacaGL % OaGaayzkaaWaaWbaaSqabeaacaaIYaaaaaaa!3C9B! {\left( {9 - \frac{x}{{40}}} \right)^2}\)(nghìn đồng). Khẳng định nào sau đây là khẳng định đúng?

Số tiền thu được khi có x HS là :

\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzaiaacI % cacaWG4bGaaiykaiabg2da9iaadIhadaqadaqaaiaaiMdacqGHsisl % daWcaaqaaiaadIhaaeaacaaI0aGaaGimaaaaaiaawIcacaGLPaaada % ahaaWcbeqaaiaaikdaaaaaaa!41DF! f(x) = x{\left( {9 - \frac{x}{{40}}} \right)^2}\)

Ta có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzaiaacE % cacaGGOaGaamiEaiaacMcacqGH9aqpdaqadaqaaiaaiMdacqGHsisl % daWcaaqaaiaadIhaaeaacaaI0aGaaGimaaaaaiaawIcacaGLPaaada % ahaaWcbeqaaiaaikdaaaGccqGHsislcaaIYaGaaiOlamaalaaabaGa % aGymaaqaaiaaisdacaaIWaaaamaabmaabaGaaGyoaiabgkHiTmaala % aabaGaamiEaaqaaiaaisdacaaIWaaaaaGaayjkaiaawMcaaiaadIha % cqGH9aqpdaqadaqaaiaaiMdacqGHsisldaWcaaqaaiaadIhaaeaaca % aI0aGaaGimaaaaaiaawIcacaGLPaaadaqadaqaaiaaiMdacqGHsisl % daWcaaqaaiaadIhaaeaacaaI0aGaaGimaaaacqGHsisldaWcaaqaai % aadIhaaeaacaaIYaGaaGimaaaaaiaawIcacaGLPaaacqGH9aqpdaqa % daqaaiaaiMdacqGHsisldaWcaaqaaiaadIhaaeaacaaI0aGaaGimaa % aaaiaawIcacaGLPaaadaqadaqaaiaaiMdacqGHsisldaWcaaqaaiaa % iodacaWG4baabaGaaGinaiaaicdaaaaacaGLOaGaayzkaaaaaa!6A21! f'(x) = {\left( {9 - \frac{x}{{40}}} \right)^2} - 2.\frac{1}{{40}}\left( {9 - \frac{x}{{40}}} \right)x = \left( {9 - \frac{x}{{40}}} \right)\left( {9 - \frac{x}{{40}} - \frac{x}{{20}}} \right) = \left( {9 - \frac{x}{{40}}} \right)\left( {9 - \frac{{3x}}{{40}}} \right)\)

\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzaiaacE % cacaGGOaGaamiEaiaacMcacqGH9aqpcaaIWaGaeyi1HS9aaeWaaeaa % caaI5aGaeyOeI0YaaSaaaeaacaWG4baabaGaaGinaiaaicdaaaaaca % GLOaGaayzkaaWaaeWaaeaacaaI5aGaeyOeI0YaaSaaaeaacaaIZaGa % amiEaaqaaiaaisdacaaIWaaaaaGaayjkaiaawMcaaiabg2da9iaaic % dacqGHuhY2daWabaabaeqabaGaamiEaiabg2da9iaaiodacaaI2aGa % aGimaaqaaiaadIhacqGH9aqpcaaIXaGaaGOmaiaaicdaaaGaay5waa % aaaa!57C1! f'(x) = 0 \Leftrightarrow \left( {9 - \frac{x}{{40}}} \right)\left( {9 - \frac{{3x}}{{40}}} \right) = 0 \Leftrightarrow \left[ \begin{array}{l} x = 360\\ x = 120 \end{array} \right.\)

\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzaiaacI % cacaaIXaGaaGOmaiaaicdacaGGPaGaeyypa0JaaGinaiaac6cacaaI % ZaGaaGOmaiaaicdacaGG7aGaamOzaiaacIcacaaIYaGaaGimaiaaic % dacaGGPaGaeyypa0JaaG4maiaac6cacaaIYaGaaGimaiaaicdaaaa!48EA! f(120) = 4.320;f(200) = 3.200\)

Vậy \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaCbeaeaaci % GGTbGaaiyyaiaacIhaaSqaaiaadIhacqGHiiIZcaqGBbGaaGimaiaa % cUdacaaIYaGaaGimaiaaicdacaGGDbaabeaakiaadAgacaGGOaGaam % iEaiaacMcacqGH9aqpcaWGMbGaaiikaiaaigdacaaIYaGaaGimaiaa % cMcacqGH9aqpcaaI0aGaaiOlaiaaiodacaaIYaGaaGimaaaa!4E58! \mathop {\max }\limits_{x \in {\rm{[}}0;200]} f(x) = f(120) = 4.320\)