Trong tất cả các cặp (x;y) thỏa mãn \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ % gacaGGNbWaaSbaaSqaaiaadIhadaahaaadbeqaaiaaikdaaaWccqGH % RaWkcaWG5bWaaWbaaWqabeaacaaIYaaaaSGaey4kaSIaaGOmaaqaba % GcdaqadaqaaiaaisdacaWG4bGaey4kaSIaaGinaiaadMhacqGHsisl % caaI0aaacaGLOaGaayzkaaGaeyyzImRaaGymaaaa!496D! {\log _{{x^2} + {y^2} + 2}}\left( {4x + 4y - 4} \right) \ge 1\). Tìm m để tồn tại duy nhất cặp (x,y) sao cho \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEamaaCa % aaleqabaGaaGOmaaaakiabgUcaRiaadMhadaahaaWcbeqaaiaaikda % aaGccqGHRaWkcaaIYaGaamiEaiabgkHiTiaaikdacaWG5bGaey4kaS % IaaGOmaiabgkHiTiaad2gacqGH9aqpcaaIWaaaaa!4536! {x^2} + {y^2} + 2x - 2y + 2 - m = 0\).

Ta có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ % gacaGGNbWaaSbaaSqaaiaadIhadaahaaadbeqaaiaaikdaaaWccqGH % RaWkcaWG5bWaaWbaaWqabeaacaaIYaaaaSGaey4kaSIaaGOmaaqaba % GcdaqadaqaaiaaisdacaWG4bGaey4kaSIaaGinaiaadMhacqGHsisl % caaI0aaacaGLOaGaayzkaaGaeyyzImRaaGymaaaa!496D! {\log _{{x^2} + {y^2} + 2}}\left( {4x + 4y - 4} \right) \ge 1\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HSTaam % iEamaaCaaaleqabaGaaGOmaaaakiabgUcaRiaadMhadaahaaWcbeqa % aiaaikdaaaGccqGHsislcaaI0aGaamiEaiabgkHiTiaaisdacaWG5b % Gaey4kaSIaaGOnaiabgsMiJkaaicdaaaa!4675! \Leftrightarrow {x^2} + {y^2} - 4x - 4y + 6 \le 0 (1)\)

Giả sử  M(x;y) thỏa mãn pt (1), khi đó tập hợp điểm M là hình tròn \((C_1)\) tâm I(2;2) bán kính \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOuamaaBa % aaleaacaaIXaaabeaakiabg2da9maakaaabaGaaGOmaaWcbeaaaaa!3999! {R_1} = \sqrt 2 \)

Các đáp án đề cho đều ứng với m > 0. Nên dễ thấy \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEamaaCa % aaleqabaGaaGOmaaaakiabgUcaRiaadMhadaahaaWcbeqaaiaaikda % aaGccqGHRaWkcaaIYaGaamiEaiabgkHiTiaaikdacaWG5bGaey4kaS % IaaGOmaiabgkHiTiaad2gacqGH9aqpcaaIWaaaaa!4536! {x^2} + {y^2} + 2x - 2y + 2 - m = 0\) là phương trình đường tròn \((C_2)\) tâm J(-1;1) bán kính \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOuamaaBa % aaleaacaaIYaaabeaakiabg2da9maakaaabaGaamyBaaWcbeaaaaa!39D0! {R_2} = \sqrt m \)

Vậy để tồn tại duy nhất cặp (x;y) thỏa đề khi chỉ khi \((C_1)\) và \((C_2)\) tiếp xúc ngoài

\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyi1HSTaam % ysaiaadQeacqGH9aqpcaWGsbWaaSbaaSqaaiaaigdaaeqaaOGaey4k % aSIaamOuamaaBaaaleaacaaIYaaabeaakiabgsDiBpaakaaabaGaaG % ymaiaaicdaaSqabaGccqGH9aqpdaGcaaqaaiaad2gaaSqabaGccqGH % RaWkdaGcaaqaaiaaikdaaSqabaGccqGHuhY2caWGTbGaeyypa0Zaae % WaaeaadaGcaaqaaiaaigdacaaIWaaaleqaaOGaeyOeI0YaaOaaaeaa % caaIYaaaleqaaaGccaGLOaGaayzkaaWaaWbaaSqabeaacaaIYaaaaa % aa!516A! \Leftrightarrow IJ = {R_1} + {R_2} \Leftrightarrow \sqrt {10} = \sqrt m + \sqrt 2 \Leftrightarrow m = {\left( {\sqrt {10} - \sqrt 2 } \right)^2}\)