已知2sina/2·cos(π/2-a/2)=3/4,求sina
问题描述：
已知2sina/2·cos(π/2-a/2)=3/4,求sina
答
2sin(A/2)·cos(π/2-A/2)=3/4
2[sin(A/2)]^2 =3/4
1-cosA= 3/4
cosA =1/4
sinA = ±√15 /4