(1) AB=AD ∠ABC=∠ADB
β=∠ABC=∠ADB=α+∠C=α+(90-β)
2β=90+α
cos2β=cos(90+α)=-sinα,sinα+cos2β=0
(2)正弦定理:AC/sin∠ADC=DC/sinα
sin∠ADC=sin(180-∠ADB)=sin(180-β)=sinβ
AC=√3*DC
得:sinβ=√3sinα=√3sin(2β-90)=-√3cos2β=-√3(1-2sin^2 β)
2√3*(sinβ)^2-sinβ -√3=0
sinβ=√3/2,β=60度