đại số 9 (bđt)

C

conga222222

$\begin{array}{l}
x + y \ge {x^2} + {y^2} \ge 0\\
dau = \leftrightarrow x = y = 0\\
\cos i:\\
{x^2} + 1 \ge 2\left| x \right| \ge 2x\\
{y^2} + 1 \ge 2\left| y \right| \ge 2y\\
\to {x^2} + {y^2} + 2 \ge 2x + 2y\\
ma\;x + y \ge {x^2} + {y^2}\\
\to x + y + 2 \ge {x^2} + {y^2} + 2 \ge 2x + 2y\\
\leftrightarrow x + y \le 2\\
dau = \leftrightarrow x = y = 1
\end{array}$
 
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