[Toán 8]Tìm GTLN, GTNN

T

thienlong_cuong

A = 2x^2+y^2+2xy-6x-4y

A = (x^2 + y^2 + 4 - 4x + 2xy - 4y) + (x^2 - 2x + 1) - 5

A = ( x + y - 2)^2 + (x -1)^2 -5 \geq -5 với \forallx

=> Min A = -5
\Leftrightarrow x + y - 2 = 0 => x + y = 2
_______________________ => x = y = 1
Và x - 1 = 0 ___ => x = 1____
 
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