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# Vectors (finding the angle) watch

1. If is the unit vector and is the unit vector , by using the cosine formula in triangle OAB find the angle between &..

I have tried expressing them as direction cosines , but none of that is working..can anyone point me in the right direction..
Thanks
2. Do you know the formula , where is the angle between a and b?
3. yes , but the book has not introduced scalar products yet..all I know is unit vector.|V|=V and direction cosines, and eq of straight lines..
4. dot/scalar product?
5. (Original post by rbnphlp)
yes , but the book has not introduced scalar products yet..all I know is unit vector.|V|=V and direction cosines, and eq of straight lines..
OK, then I can only assume you're supposed to be proving that formula.

Which is rather hard for an exercise I would have thought. See

http://en.wikipedia.org/wiki/Dot_pro...interpretation
6. (Original post by DFranklin)
OK, then I can only assume you're supposed to be proving that formula.

Which is rather hard for an exercise I would have thought. See

http://en.wikipedia.org/wiki/Dot_pro...interpretation
thanks ..this the last question in the excercise before the introduction of scalar product ..
7. am i missing something here? using cosine rule in OAB with angle between OA and OB =A
a=|AB|,b=|OB|,c=|OA|

a^2=b^2+c^20-2bc cosA

but b=c=1 as unit vectors.
also (l1)^2+(m1)^2+(n1)^2=1 and (l2)^2+(m2)^2+(n2)^2

result follows
8. (Original post by mathz)
am i missing something here? using cosine rule in OAB with angle between OA and OB =A
a=AB,b=OB,c=OA

a^2=b^2+c^20-2bc cosA

but b=c=1 as unit vectors.
also (l1)^2+(m1)^2+(n1)^2=1 and (l2)^2+(m2)^2+(n2)^2

result follows
Thanks much easier and probably the preferred method ..will rep tommorow

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