y=2x^2-4x-1 =2x^2-4x+2-3 =2(x-1)^2-3 (y+3)/2=(x-1)^2 x=-3 x-1=-√(y+3)/2 x=1-√(y+3)/2 即反函数为y=1-√(x+3)/2 定义域x>=-3