f(x)=√3/2* sin2x-1/2cos2x-1
＝sin（2x-π/6)-1
T=2π/2=π
函数的最小值=-2
2)c=√3,f(c)=0,sinB=2sinA,
f(x)=sin（2c-π/6)-1=0
2c-π/6=π/2
c=π/3
sinB=2sinA,
b=2a
c^2=a^2+b^2-2abcosπ/3
5a^2-4a^2=3
a=√3,c=√3且c=π/3
三角形是等边三角形
a=b=c=√3