Trang cá nhân của Dorae mon

1. They haven't listened to music for 3 days

It is................................................................

1. My sister hasn't written to me since she left school

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1. They haven't studied English for 7 months

Is is...............................................................

b) $12 - 6 x^{2} - 6 x = - 6 (x^{2} + x - 2)$

$= - 6 (x^{2} + 2 x . \frac{1}{2} + \frac{1}{4} - \frac{9}{4})$

$= - 6 {(x + \frac{1}{2})}^{2} + \frac{27}{2}$

Mà: ${(x + \frac{1}{2})}^{2} \geq 0$ $\Leftrightarrow - 6 {(x + \frac{1}{2})}^{2} \leq 0$

$\Leftrightarrow - 6 {(x + \frac{1}{2})}^{2} + \frac{27}{2} \leq \frac{27}{2}$ $\Leftrightarrow 12 - 6 x^{2} - 6 x \leq \frac{27}{2}$

Dấu = xảy ra khi: ${(x + \frac{1}{2})}^{2} = 0$

$\Leftrightarrow x + \frac{1}{2} = 0$ $\Leftrightarrow x = - \frac{1}{2}$ (T/m)

Vậy GTLN của 12 - 6x² - 6x là $\frac{27}{2}$ khi x = $- \frac{1}{2}$ .

13

1. I haven't worn uniform for 4 years

It is........................................................

Dorae mon