f(x)=6cos²x＋√3sin2x
=3cos2x+3+√3sin2x
=2√3(√3/2sin2x+1/2cos2x)+3
=2√3sin(2x+π/6)+3
f(a)=2√3sin(2a+π/6)+3=3-2√3
sin(2a+π/6)=-1
∵a∈(0,π/2)
∴2a+π/6∈(π/6,7π/6)
a无解已知椭圆C:x^2/a^2+y^2/b^2=1(a>b>0)的离心率为1/2，右焦点到直线l1；3x+4y=0的距离为3/5求椭圆C的方程