Cho a,b,c là các số thực sao cho phương trình \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamOEamaaCaaaleqabaGaaG4maaaakiabgUca % RiaadggacaWG6bWaaWbaaSqabeaacaaIYaaaaOGaey4kaSIaamOyai % aadQhacqGHRaWkcaWGJbGaeyypa0JaaGimaaaa!48ED! {z^3} + a{z^2} + bz + c = 0\) có ba nghiệm phức lần lượt là \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamOEamaaBaaaleaacaaIXaaabeaakiabg2da % 9iabeM8a3jabgUcaRiaaiodacaWGPbGaai4oaiaabccacaWG6bWaaS % baaSqaaiaaikdaaeqaaOGaeyypa0JaeqyYdCNaey4kaSIaaGyoaiaa % dMgacaGG7aGaaeiiaiaadQhadaWgaaWcbaGaaG4maaqabaGccqGH9a % qpcaaIYaGaeqyYdCNaeyOeI0IaaGinaaaa!5585! {z_1} = \omega + 3i;{\rm{ }}{z_2} = \omega + 9i;{\rm{ }}{z_3} = 2\omega - 4\), trong đó \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaeqyYdChaaa!3EBB! \omega \) là một số phức nào đó. Tính giá trị của \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamiuaiabg2da9maaemaabaGaamyyaiabgUca % RiaadkgacqGHRaWkcaWGJbaacaGLhWUaayjcSdGaaiOlaaaa!4716! P = \left| {a + b + c} \right|.\)
Hãy suy nghĩ và trả lời câu hỏi trước khi xem đáp án
Lời giải:
Báo saiTa có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamOEamaaBaaaleaacaaIXaaabeaakiabgUca % RiaadQhadaWgaaWcbaGaaGOmaaqabaGccqGHRaWkcaWG6bWaaSbaaS % qaaiaaiodaaeqaaOGaeyypa0JaeyOeI0IaamyyaiabgsDiBlaaisda % caWG3bGaey4kaSIaaGymaiaaikdacaWGPbGaeyOeI0IaaGinaiabg2 % da9iabgkHiTiaadggaaaa!5340! {z_1} + {z_2} + {z_3} = - a \Leftrightarrow 4w + 12i - 4 = - a\) là số thực, suy ra w có phần ảo 3i hay w = m - 3i.
Khi đó \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamOEamaaBaaaleaacaaIXaaabeaakiabg2da % 9iaad2gacaGG7aGaaGPaVlaaykW7caWG6bWaaSbaaSqaaiaaikdaae % qaaOGaeyypa0JaamyBaiabgUcaRiaaiAdacaWGPbGaai4oaiaaykW7 % caWG6bWaaSbaaSqaaiaaiodaaeqaaOGaeyypa0JaaGOmaiaad2gacq % GHsislcaaI2aGaamyAaiabgkHiTiaaisdaaaa!565B! {z_1} = m;\,\,{z_2} = m + 6i;\,{z_3} = 2m - 6i - 4\) mà \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamOEamaaBaaaleaacaaIZaaabeaakiaacUda % caaMc8UaaGPaVlaadQhadaWgaaWcbaGaaGOmaaqabaaaaa!449D! {z_3};\,\,{z_2}\) là liên hợp của nhau nên \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamyBaiabg2da9iaaikdacaWGTbGaeyOeI0Ia % aGinaiabgsDiBlaad2gacqGH9aqpcaaI0aaaaa!4752! m = 2m - 4 \Leftrightarrow m = 4\).
Vậy \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamOEamaaBaaaleaacaaIXaaabeaakiabg2da % 9iaaisdacaGG7aGaaGPaVlaaykW7caWG6bWaaSbaaSqaaiaaikdaae % qaaOGaeyypa0JaaGinaiabgUcaRiaaiAdacaWGPbGaai4oaiaaykW7 % caWG6bWaaSbaaSqaaiaaiodaaeqaaOGaeyypa0JaaGinaiabgkHiTi % aaiAdacaWGPbaaaa!5358! {z_1} = 4;\,\,{z_2} = 4 + 6i;\,{z_3} = 4 - 6i\).
Theo Viet ta có.
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaWaaiqaaqaabeqaaiaadQhadaWgaaWcbaGaaGym % aaqabaGccqGHRaWkcaWG6bWaaSbaaSqaaiaaikdaaeqaaOGaey4kaS % IaamOEamaaBaaaleaacaaIZaaabeaakiabg2da9iabgkHiTiaadgga % aeaacaWG6bWaaSbaaSqaaiaaigdaaeqaaOGaamOEamaaBaaaleaaca % aIYaaabeaakiabgUcaRiaadQhadaWgaaWcbaGaaGOmaaqabaGccaWG % 6bWaaSbaaSqaaiaaiodaaeqaaOGaey4kaSIaamOEamaaBaaaleaaca % aIXaaabeaakiaadQhadaWgaaWcbaGaaG4maaqabaGccqGH9aqpcaWG % IbaabaGaamOEamaaBaaaleaacaaIXaaabeaakiaadQhadaWgaaWcba % GaaGOmaaqabaGccaWG6bWaaSbaaSqaaiaaiodaaeqaaOGaeyypa0Ja % eyOeI0Iaam4yaaaacaGL7baacqGHshI3daGabaabaeqabaGaamyyai % abg2da9iabgkHiTiaaigdacaaIYaaabaGaamOyaiabg2da9iaaiIda % caaI0aaabaGaam4yaiabg2da9iabgkHiTiaaikdacaaIWaGaaGioaa % aacaGL7baaaaa!70D5! \left\{ \begin{array}{l} {z_1} + {z_2} + {z_3} = - a\\ {z_1}{z_2} + {z_2}{z_3} + {z_1}{z_3} = b\\ {z_1}{z_2}{z_3} = - c \end{array} \right. \Rightarrow \left\{ \begin{array}{l} a = - 12\\ b = 84\\ c = - 208 \end{array} \right.\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D % aerbdfgBPjMCPbqeeuuDJXwAKbsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGaamiuaiabg2da9maaemaabaGaeyOeI0IaaGym % aiaaikdacqGHRaWkcaaI4aGaaGinaiabgkHiTiaaikdacaaIWaGaaG % ioaaGaay5bSlaawIa7aiabg2da9iaaigdacaaIZaGaaGOnaaaa!4D14! P = \left| { - 12 + 84 - 208} \right| = 136\)
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
Tuyển chọn số 3