Ответ:

cos(ab)=

∣a∣∗∣b∣

a∗b

|a|=√((-1)²+(-1)²)=√2

|b|=√(2²+0²)=2

cos(a b)= \frac{-1*2+(-1)*0}{ \sqrt{2}*2 } =- \frac{2}{2 \sqrt{2} } =- \frac{ \sqrt{2} }{2}cos(ab)=

2

∗2

−1∗2+(−1)∗0

=−

2

2

2

=−

2

2

<(a b)=135°