Даны векторы 𝑎 → ( 2 ; 1 ) ${\displaystyle \stackrel{\to }{a}\left(2;1\right)}$ и 𝑏 → ( 2 ; − 4 ) . ${\displaystyle \stackrel{\to }{b}\left(2;-4\right).}$ Найдите скалярное произведение векторов 𝑎 → + 𝑏 → ${\displaystyle \stackrel{\to }{a}+\stackrel{\to }{b}}$ и 7 𝑎 → − 𝑏 → . ${\displaystyle 7\stackrel{\to }{a}-\stackrel{\to }{b}.}$