Alexei Averchenko
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 1d awarded Nice Answer Mar 12 awarded Notable Question Nov 21 awarded Yearling Sep 7 comment Division by $0$ @JMCF125 then you roll Aug 11 awarded Popular Question Apr 27 awarded Nice Question Jan 21 awarded Popular Question Jan 5 awarded Popular Question Nov 21 awarded Yearling Nov 13 awarded Necromancer Nov 3 awarded Favorite Question Oct 9 awarded Popular Question Sep 30 awarded Explainer Sep 24 awarded Autobiographer Aug 14 revised How can I prove that $xy\leq x^2+y^2$? added 637 characters in body Aug 14 comment How can I prove that $xy\leq x^2+y^2$? @ronno Not really, because $X := \mathbb{R}^2 \setminus \{x = -y\}$ is dense in $\mathbb{R}^2$, and $[0, +\infty)$ is closed in $\mathbb{R}$, therefore its pullback along $(x, y) \mapsto x^2 - xy + y^2$ must be $\operatorname{cl}X = \mathbb{R}^2$. Of course it is too technical for a precalculus-level question. Jul 2 awarded Curious Jul 2 awarded Inquisitive May 10 awarded Good Question Mar 30 revised How can I prove that $xy\leq x^2+y^2$? Removed dickishness