【简答题】已知 i , j 分别是与x轴,y轴正方向相同的单位向量, OB 1 = ai - 6j (a∈R),对任意n, B n B n+1 = 6i +3? 2 n-1 j . (1)若 OB 1 ⊥ B 2 B 3 ,求a的值; (2)求向量 OB n .
【判断题】在HTML中,top不是链接的目标属性。
【单选题】What did he do to make her happy.
A.
He studied much harder.
B.
He often took her for a tide.
C.
He alwaysendured her insults.
D.
He often bought her presents and meals.
【判断题】在HTML中,top不是链接的目标属性。
【单选题】已知 i 、 j 分别是与x轴、y轴方向相同的单位向量,且 AC =-3 i +6 j , BC =-6 i +4 j , BD =- i -6 j ,则一定共线的三点是( )