Trong không gian với hệ tọa độ Oxyx , cho đường thẳng \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamizaiaacQ % dadaWcaaqaaiaadIhacqGHsislcaaIXaaabaGaaGymaaaacqGH9aqp % daWcaaqaaiaadMhacqGHsislcaaIYaaabaGaaGymaaaacqGH9aqpda % WcaaqaaiaadQhacqGHsislcaaIXaaabaGaaGOmaaaaaaa!43FB! d:\frac{{x - 1}}{1} = \frac{{y - 2}}{1} = \frac{{z - 1}}{2}\),A(2;1;4) . Gọi H(a;b;c) là điểm thuộc d sao cho AH có độ dài nhỏ nhất. Tính \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamivaiabg2 % da9iaadggadaahaaWcbeqaaiaaiodaaaGccqGHRaWkcaWGIbWaaWba % aSqabeaacaaIZaaaaOGaey4kaSIaam4yamaaCaaaleqabaGaaG4maa % aaaaa!3F1D! T = {a^3} + {b^3} + {c^3}\).
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Lời giải:
Báo saiPhương trình tham số của đường thẳng d: \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaiqaaeaafa % qabeWabaaabaGaamiEaiabg2da9iaaigdacqGHRaWkcaWG0baabaGa % amyEaiabg2da9iaaikdacqGHRaWkaeaacaWG6bGaeyypa0JaaGymai % abgUcaRiaaikdacaWG0baaaaGaay5EaaGaamiDaaaa!45A7! \left\{ {\begin{array}{*{20}{c}} {x = 1 + t}\\ {y = 2 + }\\ {z = 1 + 2t} \end{array}} \right.t\)\(( t\in R)\)
\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisaiabgI % GiolaadsgacqGHshI3caWGibWaaeWaaeaacaaIXaGaey4kaSIaamiD % aiaacUdacaaMc8UaaGOmaiabgUcaRiaadshacaGG7aGaaGPaVlaaig % dacqGHRaWkcaaIYaGaamiDaaGaayjkaiaawMcaaaaa!4AF3! H \in d \Rightarrow H\left( {1 + t;\,2 + t;\,1 + 2t} \right)\)
Độ dài \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaiaadI % eacqGH9aqpdaGcaaqaamaabmaabaGaamiDaiabgkHiTiaaigdaaiaa % wIcacaGLPaaadaahaaWcbeqaaiaaikdaaaGccqGHRaWkdaqadaqaai % aadshacqGHRaWkcaaIXaaacaGLOaGaayzkaaWaaWbaaSqabeaacaaI % YaaaaOGaey4kaSYaaeWaaeaacaaIYaGaamiDaiabgkHiTiaaiodaai % aawIcacaGLPaaadaahaaWcbeqaaiaaikdaaaaabeaakiabg2da9maa % kaaabaGaaGOnaiaadshadaahaaWcbeqaaiaaikdaaaGccqGHsislca % aIXaGaaGOmaiaadshacqGHRaWkcaaIXaGaaGymaaWcbeaakiabg2da % 9maakaaabaGaaGOnamaabmaabaGaamiDaiabgkHiTiaaigdaaiaawI % cacaGLPaaadaahaaWcbeqaaiaaikdaaaGccqGHRaWkcaaI1aaaleqa % aOGaeyyzIm7aaOaaaeaacaaI1aaaleqaaaaa!5F3F! AH = \sqrt {{{\left( {t - 1} \right)}^2} + {{\left( {t + 1} \right)}^2} + {{\left( {2t - 3} \right)}^2}} = \sqrt {6{t^2} - 12t + 11} = \sqrt {6{{\left( {t - 1} \right)}^2} + 5} \ge \sqrt 5 \)
Độ dài AH nhỏ nhất bằng \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaca % aI1aaaleqaaaaa!36CD! \sqrt 5 \) khi \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiDaiabg2 % da9iaaigdacqGHshI3caWGibWaaeWaaeaacaaIYaGaai4oaiaaioda % caGG7aGaaG4maaGaayjkaiaawMcaaaaa!4114! t = 1 \Rightarrow H\left( {2;3;3} \right)\).
Vậy a = 2;b = 3; c =3; \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyO0H4Taam % yyamaaCaaaleqabaGaaG4maaaakiabgUcaRiaadkgadaahaaWcbeqa % aiaaiodaaaGccqGHRaWkcaWGJbWaaWbaaSqabeaacaaIZaaaaOGaey % ypa0JaaGOnaiaaikdaaaa!4227! \Rightarrow {a^3} + {b^3} + {c^3} = 62\)
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
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