y=2*a*sin(x)*cos(x)+2*b*cos(x)^2
f(0)=2*b=8
b=4
f(pi/6)=sqrt(3)a/2+1.5b=12
a=4sqrt(3)
解f'(x)=0得x=-pi/3,x=pi/6
检验得y=-4,y=12
取x=pi/6,y=12