Có bao nhiêu giá trị dương của số thực a sao cho phương trình \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOEamaaCa % aaleqabaGaaGOmaaaakiabgUcaRmaakaaabaGaaG4maaWcbeaakiaa % dQhacqGHRaWkcaWGHbWaaWbaaSqabeaacaaIYaaaaOGaeyOeI0IaaG % OmaiaadggacqGH9aqpcaaIWaaaaa!41B2! {z^2} + \sqrt 3 z + {a^2} - 2a = 0\) có nghiệm phức \(z_0\) thỏa mãn \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6bWaaSbaaSqaaiaaicdaaeqaaaGccaGLhWUaayjcSdGaeyypa0Za % aOaaaeaacaaIZaaaleqaaaaa!3CE2! \left| {{z_0}} \right| = \sqrt 3 \).
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Lời giải:
Báo saiTH1 : Phương trình có nghiệm thực z thỏa \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6baacaGLhWUaayjcSdGaeyypa0ZaaOaaaeaacaaIZaaaleqaaOGa % eyO0H4TaamyBaiabg2da9iaaikdacaGGUaaaaa!41C0! \left| z \right| = \sqrt 3 \Rightarrow m = 2.\)
TH2 : Phương trình không có nghiệm thực, khi đó \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyyamaaCa % aaleqabaGaaGOmaaaakiabgkHiTiaaikdacaWGHbGaeyOpa4JaaGim % aaaa!3C1D! {a^2} - 2a > 0\).
Do a là số thực nên \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOEamaaBa % aaleaacaaIXaGaaiilaiaaikdaaeqaaOGaeyypa0ZaaSaaaeaacqGH % sisldaGcaaqaaiaaiodaaSqabaGccqGHXcqScaWGPbWaaOaaaeaacq % qHuoaraSqabaaakeaacaaIYaaaaaaa!4157! {z_{1,2}} = \frac{{ - \sqrt 3 \pm i\sqrt \Delta }}{2}\) là hai số phức liên hợp của nhau
Suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6bWaaSbaaSqaaiaaigdaaeqaaaGccaGLhWUaayjcSdGaeyypa0Za % aqWaaeaacaWG6bWaaSbaaSqaaiaaikdaaeqaaaGccaGLhWUaayjcSd % aaaa!411E! \left| {{z_1}} \right| = \left| {{z_2}} \right|\), mặt khác \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaaeaaca % WG6bWaaSbaaSqaaiaaigdaaeqaaOGaaiOlaiaadQhadaWgaaWcbaGa % aGOmaaqabaaakiaawEa7caGLiWoacqGH9aqpdaabdaqaaiaadQhada % WgaaWcbaGaaGymaaqabaaakiaawEa7caGLiWoacaGGUaWaaqWaaeaa % caWG6bWaaSbaaSqaaiaaikdaaeqaaaGccaGLhWUaayjcSdGaeyypa0 % JaamyyamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaaikdacaWGHbGa % eyi1HS9aaqWaaeaacaWG6bWaaSbaaSqaaiaaigdaaeqaaaGccaGLhW % UaayjcSdGaeyypa0ZaaqWaaeaacaWG6bWaaSbaaSqaaiaaikdaaeqa % aaGccaGLhWUaayjcSdGaeyypa0ZaaOaaaeaacaWGHbWaaWbaaSqabe % aacaaIYaaaaOGaeyOeI0IaaGOmaiaadggaaSqabaaaaa!6203! \left| {{z_1}.{z_2}} \right| = \left| {{z_1}} \right|.\left| {{z_2}} \right| = {a^2} - 2a \Leftrightarrow \left| {{z_1}} \right| = \left| {{z_2}} \right| = \sqrt {{a^2} - 2a} \).
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % yyamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaaikdacaWGHbGaeyyp % a0JaaG4maiabgsDiBpaadeaaeaqabeaacaWGHbGaeyypa0JaeyOeI0 % IaaGymaaqaaiaadggacqGH9aqpcaaIZaaaaiaawUfaaaaa!4816! \Rightarrow {a^2} - 2a = 3 \Leftrightarrow \left[ \begin{array}{l} a = - 1\\ a = 3 \end{array} \right.\)
Loại a = -1 . Do đó có 2 giá trị tương đương của a thỏa mãn yêu cầu bài toán
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
Tuyển chọn số 5