[Toán 10] Giá trị lượng giác

S

shibatakeru

$\cos 3x= \cos (x+2x)\\ = \cos x.\cos 2x -\sin x \sin 2x \\= \cos x (2\cos^2x-1)-2\sin x \cos x \sin x \\ = 2\cos^3x-\cos x - 2(1-\cos^2x)\cos x = 4\cos^3x-3\cos x$
 
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