求 f(x,y) = x^3 + 3y^2 在 2x^2 + y^2 = 4 上的極大值和極小值
我用拉格朗日乘數法
令 z = x^3 + 3y^2 - L (2x^2 + y^2 - 4)
z 對 x 偏微分 3x^2 - L*4x = 0
z 對 y 偏微分 6y - L*2y = 0
z 對 L 偏微分 2x^2 + y^2 = 4
解得 x = 0 或 4, 但 x = 4 時, y 為虛數
故得 x = 0, y = 2 或 -2
當我把 (0, 2) 和 (0, -2) 代入 f(x,y)時, 其值均為 12
請問要如何判斷極大值和極小值呢?
謝謝!
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