vector{AC}=vector{AB}+vector{BC}=(3*vector{a}-4*vector{b})+(vector{a}+5*vector{b})=4*vector{a}+vector{b}
|vector{AC}|^2=vector{AC}*vector{AC}=
=(4*vector{a}+vector{b})*(4*vector{a}+vector{b})=
=16vector{a}*vector{a}+8*vector{a}*vector{b}+vector{b}*vector{b}=
=16*|vector{a}|^2+|vector{b}|^2,
vector{a}*vector{b}=|vector{a}|*|vector{b}|cos90 градусов=0
Орты - единичные векторы.
значит
|vector{AC}|^2=16*|vector{a}|^2+|vector{b}|^2=16*1+1*1=17
АС=sqrt(17)