[toán 11] Giải pt lượng giác

L

lovelycat_handoi95

[TEX]\blue{cos8x+3cos4x+3cos2x=8cosx.cos^33x-1/2[/TEX]

[TEX]\blue{PT\Leftrightarrow cos8x+3cos4x+3cos2x=2cosx(cos9x+3cos3x)-\frac{1}{2}\\ \Leftrightarrow cos8x+3cos4x+3cos2x=2cosxcos9x+6cosxcos3x-\frac{1}{2}\\ \Leftrightarrow cos8x+3cos4x+3cos2x=cos10x+cos8x+3cos4x+3cos2x-\frac{1}{2}\\\Leftrightarrow cos10x=\frac{1}{2}[/TEX]
 
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