[ Đại 10] Lượng giác

L

lp_qt

$$A= cos 382^{\circ}cos732^{\circ} + cos 248^{\circ}cos 438^{\circ} - cos 394^{\circ}
=cos22^o.cos12^o-sin22^o.sin12^o-cos34^o
=\dfrac{1}{2}(cos34^o+cos10^o)-\dfrac{1}{2}(cos10^0-cos34^o)-cos34^o
=cos34^o-cos34^o=0$$
 
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