[Toán 10] Tính

J

jelouis

$$cos12^{\circ}+cos18^{\circ}-4cos15^{\circ}cos21^{\circ}cos24^{\circ}$$
$$=cos12^{\circ}+cos18^{\circ}-2(cos36^{\circ}+cos6^{\circ})cos24^{\circ}$$
$$ =cos12^{\circ}+cos18^{\circ}-(cos60^{\circ}+cos12^{\circ}+cos30^{\circ}+cos18^{\circ})$$
$$=-(cos60^{\circ}+cos30^{\circ})=-\frac{1+\sqrt{3}}{2}$$
 
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