Cho hai số thực x,y thỏa mãn \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEamaaCa % aaleqabaGaaGOmaaaakiabgUcaRiaadMhadaahaaWcbeqaaiaaikda % aaGccqGH+aGpcaaIXaaaaa!3C7A! {x^2} + {y^2} > 1\) và \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ % gacaGGNbWaaSbaaSqaaiaadIhadaahaaadbeqaaiaaikdaaaWccqGH % RaWkcaaIYaGaamyEamaaCaaameqabaGaaGOmaaaaaSqabaGcdaqada % qaaiaaikdacaWG4bGaey4kaSIaamyEaaGaayjkaiaawMcaaiabgwMi % Zkaaigdaaaa!4620! {\log _{{x^2} + 2{y^2}}}\left( {2x + y} \right) \ge 1\). Biết giá trị lớn nhất của P = x+y là \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % WGHbGaey4kaSIaamOyamaakaaabaGaaGOnaaWcbeaaaOqaaiaadoga % aaaaaa!3A80! \frac{{a + b\sqrt 6 }}{c}\) với a,b,c là các số nguyên dương và \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % WGHbaabaGaam4yaaaaaaa!37D2! \frac{a}{c}\) tối giản. Tính S = a+ b+ c

\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ % gacaGGNbWaaSbaaSqaaiaadIhadaahaaadbeqaaiaaikdaaaWccqGH % RaWkcaaIYaGaamyEamaaCaaameqabaGaaGOmaaaaaSqabaGcdaqada % qaaiaadIhacqGHRaWkcaaIYaGaamyEaaGaayjkaiaawMcaaiabgwMi % Zkaaigdaaaa!4620! {\log _{{x^2} + 2{y^2}}}\left( {x + 2y} \right) \ge 1\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HSTaaG % OmaiaadIhacqGHRaWkcaWG5bGaeyyzImRaamiEamaaCaaaleqabaGa % aGOmaaaakiabgUcaRiaaikdacaWG5bWaaWbaaSqabeaacaaIYaaaaa % aa!4324! \Leftrightarrow 2x + y \ge {x^2} + 2{y^2}\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HS9aae % WaaeaacaWG4bGaeyOeI0IaaGymaaGaayjkaiaawMcaamaaCaaaleqa % baGaaGOmaaaakiabgUcaRiaaikdadaqadaqaaiaadMhacqGHsislda % WcaaqaaiaaigdaaeaacaaI0aaaaaGaayjkaiaawMcaamaaCaaaleqa % baGaaGOmaaaakiabgsMiJoaalaaabaGaaGyoaaqaaiaaiIdaaaaaaa!4849! \Leftrightarrow {\left( {x - 1} \right)^2} + 2{\left( {y - \frac{1}{4}} \right)^2} \le \frac{9}{8}\)

\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HSTaam % iEaiabgUcaRiaadMhacqGH9aqpcaaIXaWaaeWaaeaacaWG4bGaeyOe % I0IaaGymaaGaayjkaiaawMcaaiabgUcaRmaalaaabaGaaGymaaqaam % aakaaabaGaaGOmaaWcbeaaaaGccaGGUaWaaOaaaeaacaaIYaaaleqa % aOWaaeWaaeaacaWG5bGaeyOeI0YaaSaaaeaacaaIXaaabaGaaGinaa % aaaiaawIcacaGLPaaacqGHRaWkdaWcaaqaaiaaiwdaaeaacaaI0aaa % aaaa!4CA9! \Leftrightarrow x + y = 1\left( {x - 1} \right) + \frac{1}{{\sqrt 2 }}.\sqrt 2 \left( {y - \frac{1}{4}} \right) + \frac{5}{4}\)

\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyizIm6aaO % aaaeaadaqadaqaaiaaigdadaahaaWcbeqaaiaaikdaaaGccqGHRaWk % daqadaqaamaakaaabaGaaGOmaaWcbeaaaOGaayjkaiaawMcaamaaCa % aaleqabaGaaGOmaaaaaOGaayjkaiaawMcaamaabmaabaWaaeWaaeaa % caWG4bGaeyOeI0IaaGymaaGaayjkaiaawMcaamaaCaaaleqabaGaaG % OmaaaakiabgUcaRiaaikdadaqadaqaaiaadMhacqGHsisldaWcaaqa % aiaaigdaaeaacaaI0aaaaaGaayjkaiaawMcaamaaCaaaleqabaGaaG % OmaaaaaOGaayjkaiaawMcaaaWcbeaakiabg2da9maalaaabaGaaGyn % aaqaaiaaisdaaaaaaa!500F! \le \sqrt {\left( {{1^2} + {{\left( {\sqrt 2 } \right)}^2}} \right)\left( {{{\left( {x - 1} \right)}^2} + 2{{\left( {y - \frac{1}{4}} \right)}^2}} \right)} = \frac{5}{4}\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyizIm6aaO % aaaeaacaaIZaGaaiOlamaalaaabaGaaGyoaaqaaiaaiIdaaaaaleqa % aOGaey4kaSYaaSaaaeaacaaI1aaabaGaaGinaaaacqGH9aqpdaWcaa % qaaiaaiwdacqGHRaWkcaaIZaWaaOaaaeaacaaI2aaaleqaaaGcbaGa % aGinaaaaaaa!4258! \le \sqrt {3.\frac{9}{8}} + \frac{5}{4} = \frac{{5 + 3\sqrt 6 }}{4}\)

Suy ra S = a+ b+c =12.