Cho tam giác SOA vuông tại O có MN // SO với M,N lần lượt nằm trên cạnh SA,OA như hình vẽ bên dưới. Đặt SO =h không đổi. Khi quay hình vẽ quanh SO thì tạo thành một hình trụ nội tiếp hình nón đỉnh S có đáy là hình tròn tâm O bán kính R = OA. Tìm độ dài của MN theo h để thể tích khối trụ là lớn nhất
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Lời giải:
Báo saiĐặt M = x , ( x > 0) và OA = a, ( a > 0 ), a là hằng số.
Ta có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % WGnbGaamOtaaqaaiaadofacaWGpbaaaiabg2da9maalaaabaGaamOt % aiaadgeaaeaacaWGpbGaamyqaaaaaaa!3D9E! \frac{{MN}}{{SO}} = \frac{{NA}}{{OA}}\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % OtaiaadgeacqGH9aqpdaWcaaqaaiaad2eacaWGobGaaiOlaiaad+ea % caWGbbaabaGaam4uaiaad+eaaaaaaa!409D! \Rightarrow NA = \frac{{MN.OA}}{{SO}}\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % OtaiaadgeacqGH9aqpdaWcaaqaaiaadIhacaWGHbaabaGaamiAaaaa % aaa!3DD0! \Rightarrow NA = \frac{{xa}}{h}\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % 4taiaad6eacqGH9aqpcaWGHbGaeyOeI0YaaSaaaeaacaWG4bGaamyy % aaqaaiaadIgaaaaaaa!3FB1! \Rightarrow ON = a - \frac{{xa}}{h}\)
Khối trụ thu được có bán kính đáy bằng ON và chiều cao bằng MN .
Thể tích khối trụ là \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOvaiabg2 % da9iabec8aWjaac6cacaWGpbGaamOtamaaCaaaleqabaGaaGOmaaaa % kiaac6cacaWGnbGaamOtaaaa!3F35! V = \pi .O{N^2}.MN\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyypa0Jaeq % iWdaNaaiOlaiaadIhacaGGUaGaamyyamaaCaaaleqabaGaaGOmaaaa % kmaabmaabaWaaSaaaeaacaWGObGaeyOeI0IaamiEaaqaaiaadIgaaa % aacaGLOaGaayzkaaWaaWbaaSqabeaacaaIYaaaaaaa!4337! = \pi .x.{a^2}{\left( {\frac{{h - x}}{h}} \right)^2}\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyypa0Jaeq % iWdaNaamyyamaaCaaaleqabaGaaGOmaaaakmaalaaabaGaaGymaaqa % aiaaikdacaWGObWaaWbaaSqabeaacaaIYaaaaaaakiaaikdacaWG4b % WaaeWaaeaacaWGObGaeyOeI0IaamiEaaGaayjkaiaawMcaamaaCaaa % leqabaGaaGOmaaaaaaa!44F9! = \pi {a^2}\frac{1}{{2{h^2}}}2x{\left( {h - x} \right)^2}\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyizIm6aaS % aaaeaacqaHapaCcaWGHbWaaWbaaSqabeaacaaIYaaaaaGcbaGaaGOm % aiaadIgadaahaaWcbeqaaiaaikdaaaaaaOWaaeWaaeaadaWcaaqaai % aaikdacaWGObaabaGaaG4maaaaaiaawIcacaGLPaaadaahaaWcbeqa % aiaaiodaaaaaaa!42D4! \le \frac{{\pi {a^2}}}{{2{h^2}}}{\left( {\frac{{2h}}{3}} \right)^3}\)
Dấu bằng xảy ra khi \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGOmaiaadI % hacqGH9aqpcaWGObGaeyOeI0IaamiEaaaa!3B8A! 2x = h - x\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HSTaam % iEaiabg2da9maalaaabaGaamiAaaqaaiaaiodaaaaaaa!3C0D! \Leftrightarrow x = \frac{h}{3}\)
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