Xét các số phức z, w thỏa mãn \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6baacaGLhWUaayjcSdGaeyypa0JaaGOmaiaacYcadaabdaqaaiaa % dMgacaWG3bGaeyOeI0IaaGOmaiabgUcaRiaaiwdacaWGPbaacaGLhW % UaayjcSdGaeyypa0JaaGymaaaa!478C! \left| z \right| = 2,\left| {iw - 2 + 5i} \right| = 1\). Giá trị nhỏ nhất của \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6bWaaWbaaSqabeaacaaIYaaaaOGaeyOeI0Iaae4DaiaadQhacqGH % sislcaaI0aaacaGLhWUaayjcSdaaaa!3F99! \left| {{z^2} - {\rm{w}}z - 4} \right|\) bằng:
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Lời giải:
Báo saiTheo bài ra ta có:
+) \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6baacaGLhWUaayjcSdGaeyypa0JaaGOmaiabgkDiEdaa!3E34! \left| z \right| = 2 \Rightarrow \) Tập hợp các điểm biểu diễn số phức z là đường tròn tâm \(I_1(0;0)\) bán kính \(R_1 = 2\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WGPbaacaGLhWUaayjcSdWaaqWaaeaacaqG3bGaeyOeI0YaaSaaaeaa % caaIYaGaeyOeI0IaaGynaiaadMgaaeaacaWGPbaaaaGaay5bSlaawI % a7aiabg2da9iaaigdacqGHuhY2daabdaqaaiaabEhacqGHsisldaqa % daqaaiabgkHiTiaaiwdacqGHsislcaaIYaGaamyAaaGaayjkaiaawM % caaaGaay5bSlaawIa7aiabg2da9iaaigdaaaa!5413! \left| i \right|\left| {{\rm{w}} - \frac{{2 - 5i}}{i}} \right| = 1 \Leftrightarrow \left| {{\rm{w}} - \left( { - 5 - 2i} \right)} \right| = 1\)
Tập hợp các điểm biểu diễn số phức w là đường tròn tâm \(I_2(-5;-2)\) bán kính \(R_2=1\)
Đặt \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamivaiabg2 % da9maaemaabaGaamOEamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaa % bEhacaWG6bGaeyOeI0IaaGinaaGaay5bSlaawIa7aiabg2da9maaem % aabaGaamOEamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaabEhacaWG % 6bGaeyOeI0IaamOEaiaac6cadaqdaaqaaiaadQhaaaaacaGLhWUaay % jcSdGaeyypa0ZaaqWaaeaacaWG6baacaGLhWUaayjcSdWaaqWaaeaa % caWG6bGaeyOeI0Iaae4DaiabgkHiTmaanaaabaGaamOEaaaaaiaawE % a7caGLiWoacqGH9aqpcaaIYaWaaqWaaeaacaWG6bGaeyOeI0Iaae4D % aiabgkHiTmaanaaabaGaamOEaaaaaiaawEa7caGLiWoaaaa!6519! T = \left| {{z^2} - {\rm{w}}z - 4} \right| = \left| {{z^2} - {\rm{w}}z - z.\overline z } \right| = \left| z \right|\left| {z - {\rm{w}} - \overline z } \right| = 2\left| {z - {\rm{w}} - \overline z } \right|\)
Đặt \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOEaiabg2 % da9iaadggacqGHRaWkcaWGIbGaamyAaiaacYcadaqadaqaaiaadgga % caGGSaGaamOyaiabgIGiolabl2riHcGaayjkaiaawMcaaaaa!4340! z = a + bi,\left( {a,b \in R} \right)\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca % WG6baaaiabg2da9iaadggacqGHsislcaWGIbGaamyAaaaa!3BB2! \overline z = a - bi\)\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % OEaiabgkHiTmaanaaabaGaamOEaaaacqGH9aqpcaaIYaGaamOyaiaa % dMgaaaa!3EE4! \Rightarrow z - \overline z = 2bi\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % ivaiabg2da9iaaikdadaabdaqaaiaaikdacaWGIbGaamyAaiabgkHi % TiaadEhaaiaawEa7caGLiWoaaaa!4288! \Rightarrow T = 2\left| {2bi - w} \right|\)
Gọi M(0;2b) là điểm biểu diễn số phức 2bi, N là điểm biểu diễn số phức w.
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % ivaiabg2da9iaaikdacaWGnbGaamOtamaaBaaaleaaciGGTbGaaiyA % aiaac6gaaeqaaOGaeyi1HSTaamytaiaad6eadaWgaaWcbaGaciyBai % aacMgacaGGUbaabeaaaaa!4698! \Rightarrow T = 2M{N_{\min }} \Leftrightarrow M{N_{\min }}\)
Do \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6baacaGLhWUaayjcSdGaeyypa0JaaGOmaiabgkDiElaadggadaah % aaWcbeqaaiaaikdaaaGccqGHRaWkcaWGIbWaaWbaaSqabeaacaaIYa % aaaOGaeyypa0JaaGinaiabgsDiBlabgkHiTiaaikdacqGHKjYOcaWG % IbGaeyizImQaaGOmaiabgsDiBlabgkHiTiaaisdacqGHKjYOcaaIYa % GaamOyaiabgsMiJkaaisdaaaa!5771! \left| z \right| = 2 \Rightarrow {a^2} + {b^2} = 4 \Leftrightarrow - 2 \le b \le 2 \Leftrightarrow - 4 \le 2b \le 4\)
Tập hợp các điểm M là đoạn AB với A(-4 ; 0) B(4 ; 0)
Dựa vào hình vẽ ta thấy \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamytaiaad6 % eadaWgaaWcbaGaciyBaiaacMgacaGGUbaabeaakiabg2da9iaaisda % cqGHuhY2caWGnbWaaeWaaeaacqGHsislcaaI0aGaai4oaiabgkHiTi % aaikdaaiaawIcacaGLPaaacaGGSaGaamOtamaabmaabaGaaGimaiaa % cUdacqGHsislcaaIYaaacaGLOaGaayzkaaaaaa!4B5D! M{N_{\min }} = 4 \Leftrightarrow M\left( { - 4; - 2} \right),N\left( {0; - 2} \right)\)
Vậy \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamivamaaBa % aaleaaciGGTbGaaiyAaiaac6gaaeqaaOGaeyypa0JaaGOmaiaac6ca % caaI0aGaeyypa0JaaGioaaaa!3ECF! {T_{\min }} = 2.4 = 8\)
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
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