钝角三角形ABC
90°＜∠BAC＜180°
做CD垂直AB于D
sinBAC = sin(180°-∠DAC)=sinDAC=CD/AC
cosBAC = cos(180°-∠DAC)=-cosDAC=-AD/AC
tanBAC = tan(180°-∠DAC)=-tanDAC=-CD/AD