y=6-根号下(4-X^2)0≤根号下(4-X^2)≤2所以函数的值域[4,6]
x^2属于[0,正无穷)-x^2:(负无穷,0]4-x^2:(负无穷,4]根号下(4-x^2):[0,2]-根号下(4-x^2):[-2,0]6-根号下(4-x^2):[4,6]