Раскрываем скобки как в алгебре:
(vector{a}+vector{b}+vector{c})*(2*vector{a}-vector{b})=
=2*vector{a}*vector{a}+2vector{b}*vector{a}+2*vector{c}*vector{a}-
-vector{a}*vector{b}-vector{b}*vector{b}-vector{c}*vector{b}
По свойству скалярного произведения
vector{a}*vector{b}=|vector{a}|*|vector{b}|*cos∠ (vector{a},vector{b})
vector{a}*vector{b}=vector{b}*vector{a}
и
vector{a}*vector{a}=|vector{a}|*|vector{a}|=(|vector{a}|)^2
Поэтому
vector{a}*vector{b}=1*2*cos60^(o)=1*2*(1/2)=1
vector{a}*vector{c}=1*5*cos60^(o)=1*5*(1/2)=5/2
vector{b}*vector{c}=2*5*cos60^(o)=2*5*(1/2)=5
vector{a}*vector{a}=1*1*cos0^(o)=1*1*1=1
vector{b}*vector{b}=2*2*cos0^(o)=2*2*1=4
vector{c}*vector{c}=5*5*cos0^(o)=5*5*1=25
О т в е т. 2*1+2*1+2*(5/2) - 1 - 4 - 25= -21