vector{m} × vector{n}=(3 vector{a}+vector{b}) × (vector{a}-2 vector{b})=
=3vector{a} ×vector{a}+vector{b} ×vector{a}-6 vector{a} × vector{b}-2 vector{b} × vector{b}
(cм. скрин определение и законы векторного произведения):
vector{a} ×vector{a}=0
vector{b} ×vector{b}=0
vector{a} × vector{b}=-vector{b} ×vector{a}
Получаем:
⇒ vector{m} × vector{n}=-7 vector{a} × vector{b}
|vector{m} × vector{n}|=|-7 vector{a} × vector{b}|=7*|vector{a}|*|vector{b}|*sin(3π/4)=7*(1/sqrt(2))*3*(sqrt(2)/2)=21/2
S_( Δ)=(1/2)|vector{m} × vector{n}|=21/4=5,25