1）f(x)在R上是偶函数,则f(x)=-f(-x)
已知0≤x≤π/2时,f(x)=cos(x+π/3)-1/2；则f(x)=-f(-x)=-cos(-x+π/3)+1/2=cos(x-π/3)+1/2
2）f(31π/6)=f(π+1π/6)=f（1π/6)=-1/2