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# Vectors question watch

1. The roof of a house has a rectangular base of side four metres by eight metres. The ridge line of the roof is six metres long and one metre above the base of the roof. Calculate the acute angle between two opposite slanting edges of the roof.

How would I visualise this info? I don't know what the roof looks like and which sides go where.
2. Think of it as like a pyramid
3. (Original post by stopopups)
Think of it as like a pyramid
What does it mean by '1 metre above the base of the roof'?

4. Not sure about the angle, but I'm fairly sure that's the roof shape they mean.

E: actually, I think I have the wrong angle. But hopefully the rest of it makes sense.
E2: Okay, what actually are they talking about? That last sentence is really vague...
5. How would I display that using three dimensional vectors?
6. (Original post by TSRforum)
How would I display that using three dimensional vectors?
take on corner point to be the origin, then from there, each other point can be taken as 3d vectors relative to the origin
7. (Original post by drandy76)
take on corner point to be the origin, then from there, each other point can be taken as 3d vectors relative to the origin
So I got A = (0,0,0), B= (4,0,0), C= (4,8,0), D= (0,8,0) and V= (2,2,1) thought when I use dot product I still get the wrong answer.
8. (Original post by TSRforum)
So I got A = (0,0,0), B= (4,0,0), C= (4,8,0), D= (0,8,0) and V= (2,2,1) thought when I use dot product I still get the wrong answer.
would you mind posting your working out?
9. (Original post by TSRforum)
So I got A = (0,0,0), B= (4,0,0), C= (4,8,0), D= (0,8,0) and V= (2,2,1) thought when I use dot product I still get the wrong answer.
I think your last co-ordinate should be (2,1,1)
10. (Original post by Alexion)
I think your last co-ordinate should be (2,1,1)
Tried that one now and still got wrong answer.
I made sure both vectors were directed away from the intersection. I did dot product rule with (2,1,1) and (6,1,1)
11. (Original post by TSRforum)
Tried that one now and still got wrong answer.
I made sure both vectors were directed away from the intersection. I did dot product rule with (2,1,1) and (6,1,1)
You're mixing your co-ordinates up now (it would be [2,1,1] and [2,7,1]), but I'm not sure what exactly you're trying to do...
12. to be clear, are you using theta = cos-1 dot product/product of magnitude
13. (Original post by drandy76)
to be clear, are you using theta = cos-1 dot product/product of magnitude
Yes
14. (Original post by Alexion)
You're mixing your co-ordinates up now (it would be [2,1,1] and [2,7,1]), but I'm not sure what exactly you're trying to do...
How did you get 2,7,1? Did you do V-B? If you did C-V what angle does that work out?
15. (Original post by TSRforum)
How did you get 2,7,1? Did you do V-B?
I'm just saying that by defining your corners as XYZ (0,0,0), (4,0,0), (4,8,0) and (0,8,0) you can't have a position with XY (6,2) as that's not above the roof :/
16. (Original post by Alexion)
I'm just saying that by defining your corners as XYZ (0,0,0), (4,0,0), (4,8,0) and (0,8,0) you can't have a position with XY (6,2) as that's not above the roof :/
This is my final w/o:

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