设x=cosa y=sina 则：M=y/(x+2) = cosa/(sina+2)
cosa=Msina+2M
即：2M = cosa-Msina = √(M^2+1)sin[a+arctan(1/M)]
2M(max) = √(M^2+1)
即：4M^2 = M^2+1
M = ± √3/3
∴y/（x+2）的取值范围为：[ -√3/3,√3/3 ]