回復 28# Ling 的帖子
填充第7題
\(\begin{align}
& 2\Delta ABC=3x+4y+5z=12 \\
& 3{{x}^{2}}+2{{y}^{2}}+4yz+3{{z}^{2}}=3{{x}^{2}}+2{{\left( y+z \right)}^{2}}+{{z}^{2}} \\
& \left[ 3{{x}^{2}}+2{{\left( y+z \right)}^{2}}+{{z}^{2}} \right]\left[ {{\left( \sqrt{3} \right)}^{2}}+{{\left( 2\sqrt{2} \right)}^{2}}+{{1}^{2}} \right] \\
& \ge {{\left( 3x+4y+4z+z \right)}^{2}} \\
& 3{{x}^{2}}+2{{y}^{2}}+4yz+3{{z}^{2}}\ge 12 \\
\end{align}\)
