x=2cost
f(2cost)=3*2cost+√(4-4cos^2t)=6cost+|2sint|
sint为正 f(2cost)=6cost+2sint=√(6^2+2^2)sin(t+u)=√40sin(t+u)
sint为负 f(2cost)=6cost-2sint=-√((-6)^2+2^2)sin(t-w)=-√40sin(t-w)
正弦函数取值[-1,1]
f(x) [-√40,√40]