Nghiệm nguyên nhỏ nhất của bất phương trình \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ % gacaGGNbWaaSbaaSqaaiaaicdacaGGSaGaaGOmaaqabaGccaWG4bGa % eyOeI0IaciiBaiaac+gacaGGNbWaaSbaaSqaaiaaiwdaaeqaaOWaae % WaaeaacaWG4bGaeyOeI0IaaGOmaaGaayjkaiaawMcaaiabgYda8iGa % cYgacaGGVbGaai4zamaaBaaaleaacaaIWaGaaiilaiaaikdaaeqaaO % GaaG4maaaa!4BEA! {\log _{0,2}}x - {\log _5}\left( {x - 2} \right) < {\log _{0,2}}3\) là:

[Phương pháp tự luận]

Điều kiện: x > 2

\(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ % gacaGGNbWaaSbaaSqaaiaaicdacaGGSaGaaGOmaaqabaGccaWG4bGa % eyOeI0IaciiBaiaac+gacaGGNbWaaSbaaSqaaiaaiwdaaeqaaOWaae % WaaeaacaWG4bGaeyOeI0IaaGOmaaGaayjkaiaawMcaaiabgYda8iGa % cYgacaGGVbGaai4zamaaBaaaleaacaaIWaGaaiilaiaaikdaaeqaaO % GaaG4maiabgsDiBlGacYgacaGGVbGaai4zamaaBaaaleaacaaIWaGa % aiilaiaaikdaaeqaaOWaamWaaeaacaWG4bWaaeWaaeaacaWG4bGaey % OeI0IaaGOmaaGaayjkaiaawMcaaaGaay5waiaaw2faaiabgYda8iGa % cYgacaGGVbGaai4zamaaBaaaleaacaaIWaGaaiilaiaaikdaaeqaaO % GaaG4maiabgsDiBlaadIhadaahaaWcbeqaaiaaikdaaaGccqGHsisl % caaIYaGaamiEaiabgkHiTiaaiodacqGH+aGpcaaIWaGaeyi1HS9aam % qaaqaabeqaaiaadIhacqGH8aapcqGHsislcaaIXaaabaGaamiEaiab % g6da+iaaiodaaaGaay5waaaaaa!75A4! {\log _{0,2}}x - {\log _5}\left( {x - 2} \right) < {\log _{0,2}}3 \Leftrightarrow {\log _{0,2}}\left[ {x\left( {x - 2} \right)} \right] < {\log _{0,2}}3 \Leftrightarrow {x^2} - 2x - 3 > 0 \Leftrightarrow \left[ \begin{array}{l} x < - 1\\ x > 3 \end{array} \right.\)

So điều kiện suy ra : x > 3. Chọn B

[Phương pháp trắc nghiệm]

Nhập vào màn hình máy tính \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ % gacaGGNbWaaSbaaSqaaiaaicdacaGGSaGaaGOmaaqabaGccaWGybGa % eyOeI0IaciiBaiaac+gacaGGNbWaaSbaaSqaaiaaiwdaaeqaaOWaae % WaaeaacaWGybGaeyOeI0IaaGOmaaGaayjkaiaawMcaaiabgkHiTiGa % cYgacaGGVbGaai4zamaaBaaaleaacaaIWaGaaiilaiaaikdaaeqaaO % GaaG4maaaa!4B94! {\log _{0,2}}X - {\log _5}\left( {X - 2} \right) - {\log _{0,2}}3\)

Nhấn CALC và cho X = 3 (nhỏ nhất) máy tính hiển thị 0. Vậy loại đáp án B.

Nhấn CALC và cho X = 4 máy tính hiển thị -0.6094234797. Chọn B