=vector{a}*vector{a}+ vector{b}* vector{b}+vector{c}*vector{c}+
+2*vector{a}vector{b}-2vector{b}*vector{c}-2vector{a}vector{c}=
=1*1+2*2+3*3+2*1*2*cos90^(o)-2*2*3*cos60^(o)-2*1*3*cos120^(o)=
=1+4+9+4*0-12*(1/2)-6*(-1/2)=11
|vector{a}+ vector{b}-vector{c}|=sqrt(11)