$C_{ABCD}=AB+BD+CD+AC=3R+BD+CD$ max khi$\Leftrightarrow BD+CD$ maxmà $BD+CD\geq 2\sqrt{BD.CD}$ xảy ra $\Leftrightarrow BD=CD$$\Rightarrow D$ nằm chính giữa cung $\widehat{BC}$
$IV) $ $4.$$C_{ABCD}=AB+BD+CD+AC=3R+BD+CD$ max khi$\Leftrightarrow BD+CD$ maxmà $BD+CD\geq 2\sqrt{BD.CD}$ xảy ra $\Leftrightarrow BD=CD$$\Rightarrow D$ nằm chính giữa cung $\widehat{BC}$