Xét các số phức z, w thỏa mãn \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % qG3bGaeyOeI0IaamyAaaGaay5bSlaawIa7aiabg2da9iaaikdacaGG % SaGaamOEaiabgUcaRiaaikdacqGH9aqpcaWGPbGaae4Daaaa!43E8! \left| {{\rm{w}} - i} \right| = 2,z + 2 = i{\rm{w}}\). Gọi \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOEamaaBa % aaleaacaaIXaaabeaakiaacYcacaWG6bWaaSbaaSqaaiaaikdaaeqa % aaaa!3A7B! {z_1},{z_2}\) lần lượt là các số phức mà tại đó |z| đạt giá trị nhỏ nhất và giá trị lớn nhất. Môđun \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6bWaaSbaaSqaaiaaigdaaeqaaOGaey4kaSIaamOEamaaBaaaleaa % caaIYaaabeaaaOGaay5bSlaawIa7aaaa!3DD9! \left| {{z_1} + {z_2}} \right|\) bằng:
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Lời giải:
Báo saiTheo bài ra ta có:
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaWG6b % Gaey4kaSIaaGOmaiabg2da9iaadMgacaWG3bGaeyO0H4Taam4Daiab % g2da9maalaaabaGaamOEaiabgUcaRiaaikdaaeaacaWGPbaaaaqaam % aaemaabaGaam4DaiabgkHiTiaadMgaaiaawEa7caGLiWoacqGH9aqp % caaIYaGaeyO0H49aaqWaaeaadaWcaaqaaiaadQhacqGHRaWkcaaIYa % aabaGaamyAaaaacqGHsislcaWGPbaacaGLhWUaayjcSdGaeyypa0Ja % aGOmaiabgsDiBpaaemaabaGaamOEaiabgUcaRiaaikdacqGHRaWkca % aIXaaacaGLhWUaayjcSdGaeyypa0JaaGOmaiabgsDiBpaaemaabaGa % amOEaiabgUcaRiaaiodaaiaawEa7caGLiWoacqGH9aqpcaaIYaaaaa % a!6D4C! \begin{array}{l} z + 2 = iw \Rightarrow w = \frac{{z + 2}}{i}\\ \left| {w - i} \right| = 2 \Rightarrow \left| {\frac{{z + 2}}{i} - i} \right| = 2 \Leftrightarrow \left| {z + 2 + 1} \right| = 2 \Leftrightarrow \left| {z + 3} \right| = 2 \end{array}\)
Tập hợp các điểm biểu diễn số phức z là đường tròn I(-3;0) bán kính R = 2
Gọi M là điểm biểu diễn số phức z, dựa vào hình vẽ ta có:
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaiqaaqaabe % qaamaaemaabaGaamOEaaGaay5bSlaawIa7amaaBaaaleaaciGGTbGa % aiyAaiaac6gaaeqaaOGaeyi1HSTaam4taiaad2eadaWgaaWcbaGaci % yBaiaacMgacaGGUbaabeaakiabgsDiBlaad2eadaqadaqaaiabgkHi % TiaaigdacaGG7aGaaGimaaGaayjkaiaawMcaaiabgkDiElaadQhada % WgaaWcbaGaaGymaaqabaGccqGH9aqpcqGHsislcaaIXaaabaWaaqWa % aeaacaWG6baacaGLhWUaayjcSdWaaSbaaSqaaiGac2gacaGGHbGaai % iEaaqabaGccqGHuhY2caWGpbGaamytamaaBaaaleaaciGGTbGaaiyy % aiaacIhaaeqaaOGaeyi1HSTaamytamaabmaabaGaeyOeI0IaaGynai % aacUdacaaIWaaacaGLOaGaayzkaaGaeyO0H4TaamOEamaaBaaaleaa % caaIYaaabeaakiabg2da9iabgkHiTiaaiwdaaaGaay5EaaGaeyO0H4 % 9aaqWaaeaacaWG6bWaaSbaaSqaaiaaigdaaeqaaOGaey4kaSIaamOE % amaaBaaaleaacaaIYaaabeaaaOGaay5bSlaawIa7aiabg2da9iaaiA % daaaa!7D36! \left\{ \begin{array}{l} {\left| z \right|_{\min }} \Leftrightarrow O{M_{\min }} \Leftrightarrow M\left( { - 1;0} \right) \Rightarrow {z_1} = - 1\\ {\left| z \right|_{\max }} \Leftrightarrow O{M_{\max }} \Leftrightarrow M\left( { - 5;0} \right) \Rightarrow {z_2} = - 5 \end{array} \right. \Rightarrow \left| {{z_1} + {z_2}} \right| = 6\)
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
Tuyển chọn số 4