f(x)=sin2x+acos2x
f(x)=√a^2+1sin(2x+φ)(tanφ=a)
g(x)=√a^2+1sin(2x+φ+2兀/3),要得到偶函数,只要满足φ+2兀/3=±兀/2都成立.φ=-兀/6或-7兀/6
a=tanφ=-√3/3