设f(x)=6cosx的平方-3的开根*sin2x

f(x)=3cos2x+3-√3×sin2x
=2√3(√3/2×cos2x-1/2×sin2x)+3
=2√3(sinπ/3cos2x-cosπ/3sin2x)+3
=2√3sin(π/3-2x)+3
最大值=3+2√3
最小正周期是π
f(a)=3-2√3
π/3-2a=2kπ-π/2
α是锐角
α=5π/12
4α/5=π/3
tan(4α/5)=tanπ/3=√3