векторы
CD
и
DE
через векторы
AB=a и
BC=b
vector{АB}+vector{BC}+vector{CD}+vector{DA}=0
vector{a}+vector{b}+vector{CD}-2vector{b}=0 ⇒[b] vector{CD}[/b]=2vector{b}-vector{a}-vector{b}=[b]vector{b}-vector{a}[/b]
[b]vector{DE}[/b]=-vector{AB}=[b]-vector{a} [/b]