万能公式,sin2b=2tgb/(1+tg^2b),cos2b=(1-tg^2b)/(1+tg^2b)代入=〉tgb=-1+根号3或-1-根号3因为a=60=> 0<b<b => 0< tgb < 正无穷,或 -根号3<tgb<0=> tgb = -1+根号3