Cách làm cho bạn:
Ta có:
$\Delta EAD=\Delta EDC$; $\Delta EAD=\Delta ECB$; $\Delta EAD=\Delta EBA$;
$\Delta EDC=\Delta ECB$; $\Delta EDC=\Delta EDA$; $\Delta ECB=\Delta EBA$;
$\Delta EAD=\Delta ECD$; $\Delta EAD=\Delta EBC$; $\Delta EAD=\Delta EAB$;
$\Delta EDC=\Delta EBC$; $\Delta EDC=\Delta EDA$; $\Delta ECB=\Delta EAB$;
$\Delta EAD=\Delta EDA$; $\Delta EDC=\Delta ECD$; $\Delta ECB=\Delta EBC$;$\Delta EBA=\Delta EAB$;
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