⑴I(物主代词)____ ⑵sweater(复数形式)____
⑶nurse(同类词)____ ⑷your(主格)____
⑸black(反义词)____ ⑹they(单数形式)____
(Ⅰ)求 (2a→−b→)⋅(a→+3b→) 的值;
(Ⅱ)当实数x为何值时, xa→−b→ 与 a→+3b→ 垂直?