若向量a＝(1，λ，2)，b＝(2，－1,2)，a、b的夹角的余弦值为8/9，则λ的值为( )
A．2
B．－2
C．－2或2/55
D．2或－2/55
答案：C
分析：用向量的内积公式建立方程，本题中知道了夹角的余弦值为8/9，故应用内积公式的变形来建立关于参数λ的方程求λ．
解：由题意向量
=（1，λ，2），
=（2，-1，2），且与的夹角余弦值为
，
故有cos＜，＞=
=
=，
解得：λ=-2或
．
故应选C．
根据对话选词填空。
NICK: I know, but I 2______ help Trig than go to bed. He’s tearing up his English exercise book and he’s throwing his grammar book round the room. I don’t think he’s very happy. JENNY: Well, you 3______ stay up too long. NICK: Trig, stop it. You 4______ throw your books at the wall. It won’t help and you might break something. You 5______ work at your English now. What are you learning? TRIG: Abbreviation [n] 1 U abbreviating, being abbreviated 2 C shortened form of a word, phrase, etc. ‘Sept’ an abbreviation for ‘September’… NICK: But Trig, that’s from the dictionary. You 6______ learn the whole dictionary by heart! |