Trong không gian Oxyz , cho ba điểm \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqamaabm % aabaGaaGimaiaacUdacaaIWaGaai4oaiaaigdaaiaawIcacaGLPaaa % caGGSaGaamOqamaabmaabaGaeyOeI0IaaGymaiaacUdacaaIXaGaai % 4oaiaaicdaaiaawIcacaGLPaaacaGGSaGaam4qamaabmaabaGaaGym % aiaacUdacaaIWaGaai4oaiabgkHiTiaaigdaaiaawIcacaGLPaaaaa % a!4B26! A\left( {0;0;1} \right),B\left( { - 1;1;0} \right),C\left( {1;0; - 1} \right)\). Điểm M thuộc mặt phẳng \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaaca % WGqbaacaGLOaGaayzkaaGaaiOoaiaaikdacaWG4bGaey4kaSIaaGOm % aiaadMhacqGHsislcaWG6bGaey4kaSIaaGOmaiabg2da9iaaicdaaa % a!42AE! \left( P \right):2x + 2y - z + 2 = 0\) sao cho \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaG4maiaad2 % eacaWGbbWaaWbaaSqabeaacaaIYaaaaOGaey4kaSIaaGOmaiaad2ea % caWGcbWaaWbaaSqabeaacaaIYaaaaOGaey4kaSIaamytaiaadoeada % ahaaWcbeqaaiaaikdaaaaaaa!40CA! 3M{A^2} + 2M{B^2} + M{C^2}\) đạt giá trị nhỏ nhất. Giá trị nhỏ nhất đó bằng
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Lời giải:
Báo sai
Gọi I(x,y,z) thỏa mãn \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaG4mamaaFi % aabaGaamysaiaadgeaaiaawEniaiabgUcaRiaaikdadaWhcaqaaiaa % dMeacaWGcbaacaGLxdcacqGHRaWkdaWhcaqaaiaadMeacaWGdbaaca % GLxdcacqGH9aqpdaWhcaqaaiaaicdaaiaawEniaiabgkDiElaadMea % daqadaqaaiabgkHiTmaalaaabaGaaGymaaqaaiaaiAdaaaGaai4oam % aalaaabaGaaGymaaqaaiaaiodaaaGaai4oamaalaaabaGaaGymaaqa % aiaaiodaaaaacaGLOaGaayzkaaaaaa!5239! 3\overrightarrow {IA} + 2\overrightarrow {IB} + \overrightarrow {IC} = \overrightarrow 0 \Rightarrow I\left( { - \frac{1}{6};\frac{1}{3};\frac{1}{3}} \right)\)
Ta có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiuaiabg2 % da9iaaiodacaWGnbGaamyqamaaCaaaleqabaGaaGOmaaaakiabgUca % RiaaikdacaWGnbGaamOqamaaCaaaleqabaGaaGOmaaaakiabgUcaRi % aad2eacaWGdbWaaWbaaSqabeaacaaIYaaaaOGaeyypa0JaaG4mamaa % bmaabaWaa8HaaeaacaWGnbGaamysaaGaay51GaGaey4kaSYaa8Haae % aacaWGjbGaamyqaaGaay51GaaacaGLOaGaayzkaaWaaWbaaSqabeaa % caaIYaaaaOGaey4kaSIaaGOmamaabmaabaWaa8HaaeaacaWGnbGaam % ysaaGaay51GaGaey4kaSYaa8HaaeaacaWGjbGaamOqaaGaay51Gaaa % caGLOaGaayzkaaWaaWbaaSqabeaacaaIYaaaaOGaey4kaSYaaeWaae % aadaWhcaqaaiaad2eacaWGjbaacaGLxdcacqGHRaWkdaWhcaqaaiaa % dMeacaWGdbaacaGLxdcaaiaawIcacaGLPaaadaahaaWcbeqaaiaaik % daaaaaaa!64D9! P = 3M{A^2} + 2M{B^2} + M{C^2} = 3{\left( {\overrightarrow {MI} + \overrightarrow {IA} } \right)^2} + 2{\left( {\overrightarrow {MI} + \overrightarrow {IB} } \right)^2} + {\left( {\overrightarrow {MI} + \overrightarrow {IC} } \right)^2}\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyypa0JaaG % Onaiaad2eacaWGjbWaaWbaaSqabeaacaaIYaaaaOGaey4kaSIaaGOm % amaaFiaabaGaamytaiaadMeaaiaawEniaiaac6cadaqadaqaaiaaio % dadaWhcaqaaiaadMeacaWGbbaacaGLxdcacqGHRaWkcaaIYaWaa8Ha % aeaacaWGjbGaamOqaaGaay51GaGaey4kaSYaa8HaaeaacaWGjbGaam % 4qaaGaay51GaaacaGLOaGaayzkaaGaey4kaSIaaG4maiaadMeacaWG % bbWaaWbaaSqabeaacaaIYaaaaOGaey4kaSIaaGOmaiaadMeacaWGcb % WaaWbaaSqabeaacaaIYaaaaOGaey4kaSIaamysaiaadoeadaahaaWc % beqaaiaaikdaaaGccqGH9aqpcaaI2aGaamytaiaadMeadaahaaWcbe % qaaiaaikdaaaGccqGHRaWkdaagaaqaaiaaiodacaWGjbGaamyqamaa % CaaaleqabaGaaGOmaaaakiabgUcaRiaaikdacaWGjbGaamOqamaaCa % aaleqabaGaaGOmaaaakiabgUcaRiaadMeacaWGdbWaaWbaaSqabeaa % caaIYaaaaaqaaiaadogacaWGVbGaamOBaiaadohacaWG0baakiaawI % J-aaaa!7103! = 6M{I^2} + 2\overrightarrow {MI} .\left( {3\overrightarrow {IA} + 2\overrightarrow {IB} + \overrightarrow {IC} } \right) + 3I{A^2} + 2I{B^2} + I{C^2} = 6M{I^2} + \underbrace {3I{A^2} + 2I{B^2} + I{C^2}}_{const}\)
Suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiuamaaBa % aaleaaciGGTbGaaiyAaiaac6gaaeqaaOGaeyi1HSTaamytaiaadMea % daWgaaWcbaGaciyBaiaacMgacaGGUbaabeaaaaa!40CA! {P_{\min }} \Leftrightarrow M{I_{\min }}\) hay M là hình chiếu của I trên \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaaca % WGqbaacaGLOaGaayzkaaGaeyO0H4TaamytaiaadMeadaWgaaWcbaGa % ciyBaiaacMgacaGGUbaabeaakiabg2da9iaadsgadaWadaqaaiaadM % eacaGG7aWaaeWaaeaacaWGqbaacaGLOaGaayzkaaaacaGLBbGaayzx % aaGaeyypa0ZaaSaaaeaacaaIYaaabaGaaG4maaaaaaa!49B1! \left( P \right) \Rightarrow M{I_{\min }} = d\left[ {I;\left( P \right)} \right] = \frac{2}{3}\).
Vậy \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiuamaaBa % aaleaaciGGTbGaaiyAaiaac6gaaeqaaOGaeyypa0JaaGOnaiaac6ca % daqadaqaamaalaaabaGaaGOmaaqaaiaaiodaaaaacaGLOaGaayzkaa % WaaWbaaSqabeaacaaIYaaaaOGaey4kaSIaaG4maiaac6cadaWcaaqa % aiaaiEdaaeaacaaIXaGaaGOmaaaacqGHRaWkcaaIYaGaaiOlamaala % aabaGaaGynaaqaaiaaikdaaaGaey4kaSYaaSaaaeaacaaIXaGaaG4m % aaqaaiaaisdaaaGaeyypa0ZaaSaaaeaacaaI2aGaaGymaaqaaiaaiA % daaaaaaa!4F3A! {P_{\min }} = 6.{\left( {\frac{2}{3}} \right)^2} + 3.\frac{7}{{12}} + 2.\frac{5}{2} + \frac{{13}}{4} = \frac{{61}}{6}\).
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
Tuyển chọn số 5