1.In part C of the question,the the solution at the back:
if DE // AB , (1+t)/3=(4-t)/4 ,<-Why?
2.when calculating dot product,why sometimes can use the method of expansion,while sometimes can use the definition of dot product?Take Q.9 as an example,in part(ai),the difinition is used,but in other parts of the question,"expand" method is used.why?
HKCEE A math paper 1 Q 9c
2008-03-10 1:41 am
回答 (3)
2008-03-12 12:12 am
✔ 最佳答案
A.Maths 1998 Paper 1 Q9AB = OB - OA = b - a
From the above steps, you can find that OD = (4-t)/4 a and OE = (1+2t)/3 b
Then DE = OE - OD = (1+t)/3 b - (4-t)/4 a
There is a theorem that
If u1a + v1b is parallel to u2a + v2b,
Then u1/u2 = v1/v2
Hence, [(1+t)/3] / 1 = [ (4-t)/4 ] / 1
i.e. (1+t)/3=(4-t)/4
2. Definition of dot product a.b
= |a| |b| cos θ
Of course, if you know what θ is, magnitude of a and b, you can use this definition. If not, how can you use it? Only expansion can be used.
Note: For dot products, usually |a| and |b| are given.
In part(a)(i), θ is also given. So use the definition.
If θ is not given, use expansion.
2008-03-12 1:51 am
I am sorry, it is a question from 1998 A math paper 1.
2008-03-11 6:26 am
邊年啊大佬
收錄日期: 2021-04-13 15:15:48
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20080309000051KK02438