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+$y=(2sinx-1)^2+2 \ge 2$

Dấu = xảy ra \Leftrightarrow $sinx=\dfrac{1}{2}$

+$-1 \le sinx \le 1$

\Rightarrow $-3 \le 2sinx-1 \le 1$

\Rightarrow $(2sinx-1)^2 \le 9$

\Rightarrow $y \le 11$

Dấu = xảy ra \Leftrightarrow $sinx=-1$
 
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