[toán 11] bt

N

newstarinsky

PT trở thành
$cosx+cos3x+cos4x+cos8x=0\\
\Leftrightarrow cosx+cos8x+cos3x+cos4x=0\\
\Leftrightarrow cos\dfrac{9x}{2}.cos\dfrac{7x}{2}+cos\dfrac{7x}{2}.cos\dfrac{x}{2}=0\\
\Leftrightarrow cos\dfrac{7x}{2}(cos\dfrac{9x}{2}+cos\dfrac{x}{2})=0\\
\Leftrightarrow cos\dfrac{7x}{2}.cos\dfrac{5x}{2}.cos2x=0$
 
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