Let , a, b and c be the length of three sides of triangle, represented in terms of vectors as [tex]\vec{a},\vec{b},\vec{c}[/tex].
Now, vector of same Magnitude acts as normal vector to each side.
So, equation of any vector p having normal q is given by
[tex]\vec{p} \times\vec{q}[/tex]
Now sum of three vector and it's normal is given as
[tex]=\vec{a} \times\vec{a}+\vec{b} \times\vec{b}+\vec{c} \times\vec{c}\\\\=0+0+0\\\\=0[/tex]
Cross product of two identical vectors is Zero.
[tex]\rightarrow \vec{i} \times\vec{i}=0[/tex]