Tìm m để hàm số \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOzamaabm % aabaGaamiEaaGaayjkaiaawMcaaiabg2da9maalaaabaGaamyBaiaa % dIhacqGHRaWkcaaI1aaabaGaamiEaiabgkHiTiaad2gaaaaaaa!40E7! f\left( x \right) = \frac{{mx + 5}}{{x - m}}\) đạt giá trị nhỏ nhất trên đoạn [0;1] bằng -7.
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Lời giải:
Báo saiTXĐ: \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Lq-Jirpepeea0-as0Fb9pgea0lXxe9vr0-vr % 0-vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaiaadseacqGH9a % qpcqWIDesOcaGGCbWaaiWaaeaacaWGTbaacaGL7bGaayzFaaaaaa!3DA8! D = R\backslash \left \{ m \right\}\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Lq-Jirpepeea0-as0Fb9pgea0lXxe9vr0-vr % 0-vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaiqadAgagaqbam % aabmaabaGaamiEaaGaayjkaiaawMcaaiabg2da9maalaaabaGaeyOe % I0IaamyBamaaCaaaleqabaGaaGOmaaaakiabgkHiTiaaiwdaaeaada % qadaqaaiaadIhacqGHsislcaWGTbaacaGLOaGaayzkaaaaaiabgYda % 8iaaicdacqGHaiIicaWG4bGaeyicI4Saamiraaaa!49B4! f'\left( x \right) = \frac{{ - {m^2} - 5}}{{\left( {x - m} \right)}} < 0\forall x \in D \)nên f(x) nghịch biến trên D.
Do đó \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Lq-Jirpepeea0-as0Fb9pgea0lXxe9vr0-vr % 0-vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaamaaxababaGaci % yBaiaacMgacaGGUbaaleaadaWadaqaaiaaicdacaGG7aGaaGymaaGa % ay5waiaaw2faaaqabaGccaWGMbWaaeWaaeaacaWG4baacaGLOaGaay % zkaaGaeyypa0JaamOzamaabmaabaGaaGymaaGaayjkaiaawMcaaiab % g2da9iabgkHiTiaaiEdacqGHuhY2daWcaaqaaiaad2gacqGHRaWkca % aI1aaabaGaaGymaiabgkHiTiaad2gaaaGaeyypa0JaeyOeI0IaaG4n % aiabgsDiBlaad2gacqGH9aqpcaaIYaaaaa!5758! \mathop {\min }\limits_{\left[ {0;1} \right]} f\left( x \right) = f\left( 1 \right) = - 7 \Leftrightarrow \frac{{m + 5}}{{1 - m}} = - 7 \Leftrightarrow m = 2\)