Câu 69 Trang 36: So sánh

Lớp 9 SGK Toán tập 1

Ta có :

a. $5=\sqrt[3]{5^{3}}=\sqrt[3]{125}$

Mà : $\sqrt[3]{123}<\sqrt[3]{125}$

=> $5>\sqrt[3]{123}$

b. $5\sqrt[3]{6}=\sqrt[3]{5^{3}}.\sqrt[3]{6}=\sqrt[3]{125.6}=\sqrt[3]{750}$

$6\sqrt[3]{5}=\sqrt[3]{6^{3}}.\sqrt[3]{5}=\sqrt[3]{216.5}=\sqrt[3]{1080}$

Mà : $\sqrt[3]{750}<\sqrt[3]{1080}$

=> $5\sqrt[3]{6}<6\sqrt[3]{5}$

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