运用万能公式设tan(A/2)=t
sinA=2t/(1+t^2)
cosA=(1-t^2)/(1+t^2)
代入化简得y=(t-1)^2/(3t^2+1)
y'=2(t-1)(3t-1)/(3t^2+1)^2
t=1/3时有最大值1/3
t=1时有最小值0