[tex]cos3x+\sqrt{2-cos^2 3x}=2(1+sin^2 2x)[/tex]

C

conga222222

$\eqalign{
& \cos i: \cr
& {\cos ^2}3x + 1 \ge 2\left| {\cos 3x} \right| \ge 2\cos 3x \cr
& 1 + \left( {2 - {{\cos }^2}3x} \right) \ge 2\sqrt {2 - {{\cos }^2}3x} \cr
& \to 2\cos 3x + 2\sqrt {2 - {{\cos }^2}3x} \le {\cos ^2}3x + 1 + 1 + 2 - {\cos ^2}3x = 4 \cr
& \leftrightarrow \cos 3x + \sqrt {2 - {{\cos }^2}3x} \le 2 \cr
& 2\left( {1 + {{\sin }^2}2x} \right) \ge 2 \cr
& \to VT \le VT \cr
& dau = \leftrightarrow ... \cr} $
 
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