3x^2 + 2y^2 = 6x
y^2 = -1/2 *(3x^2 - 6x)
故 x^2 + y^2 = x^2 - 1/2 *(3x^2 - 6x)
=-1/2*x^2 +3x
=-1/2(x-3)^2 + 9/2
故最大值 9/2
无最小值