sin15cos165=sin15*[-cos(180-165)]=-sin15cos15=-(1/2)2sin15cos15=-(1/2)sin30=-(1/2)(1/2)=-1/4---------------2sinA*cosA=sin2A
cos165°=-cos15°sin15°cos165°=-1/2(2sin15°cos15°)=-1/2sin30°=-1/4
(根号6-根号2)/4
