TA CĨ X2(X2 + 2Y2 – 3) + (Y2 – 2)2 = 1 ⇔ (X2 + Y2)2 – 4(X2 + Y2) +...

65. Ta cĩ x

²

(x

²

+ 2y

²

– 3) + (y

²

– 2)

²

= 1 ⇔ (x

²

+ y

²

)

²

– 4(x

²

+ y

²

) + 3 = - x

²

≤ 0.

Do đĩ : A

²

– 4A + 3 ≤ 0 ⇔ (A – 1)(A – 3) ≤ 0 ⇔ 1 ≤ A ≤ 3.

min A = 1 ⇔ x = 0, khi đĩ y = ± 1. max A = 3 ⇔ x = 0, khi đĩ y = ±

3

.