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Tìm GTNN hoặc GTLN biết :
a) M = [TEX]5x^2[/TEX] - 4xy + [TEX]y^2[/TEX] + 6x + 10
b) N = [TEX]2x^2[/TEX] + [TEX]2y^2[/TEX] - 2xy - 2x - 8y + 21
c) P = [TEX]5x^2[/TEX] + [TEX]2y^2[/TEX] - 4xy - 2x - 2y + 15
d) Q = [TEX]3x^2[/TEX] + [TEX]5y^2[/TEX] + 4xy - 14x - 2y + 27
a) M = [TEX]5x^2[/TEX] - 4xy + [TEX]y^2[/TEX] + 6x + 10
b) N = [TEX]2x^2[/TEX] + [TEX]2y^2[/TEX] - 2xy - 2x - 8y + 21
c) P = [TEX]5x^2[/TEX] + [TEX]2y^2[/TEX] - 4xy - 2x - 2y + 15
d) Q = [TEX]3x^2[/TEX] + [TEX]5y^2[/TEX] + 4xy - 14x - 2y + 27