感謝解答
倍角公式:sin2θ = 2sinθcosθcos2θ = cos2θ−sin2θ =2cos2θ−1 =1−2sin2θ本題:(A)2sin20°cos20° = sin(2*20)= sin40 = cos50(B)cos 2 35° - sin2 35° = cos(2*35)= cos70(C)2cos2 40° - 1 = cos(2*40)= cos80(D)2sin2 70° - 1 = - cos(2*70) = -cos140 = cos 40 cosθ在第一象限角度越接近90度越小,故答案為C
17.下列選項中何者的值最小﹖ (A) 2sin20°cos20° (B)..-阿摩線上測驗