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# Vectors (C4) watch

1. The points A and B have coordinates (2, 1, -1) and (3, 1, -2) respectively.
The angle OBA is θ, where O is the origin.
a)
(i) Find the vector AB
(ii) Show that cosθ=5/(2√7)

The point C is such that OC =2OB. The line l is parallel to AB and passes through the point C. Find the vector equation of l.
The point D lies on l such that the angle ODC = 90o. Find the coordinates of D.
2. (Original post by katiealex99)
The points A and B have coordinates (2, 1, -1) and (3, 1, -2) respectively.
The angle OBA is θ, where O is the origin.
a)
(i) Find the vector AB
(ii) Show that cosθ=5/(2√7)

The point C is such that OC =2OB. The line l is parallel to AB and passes through the point C. Find the vector equation of l.
The point D lies on l such that the angle ODC = 90o. Find the coordinates of D.
Interesting, how much of that can you do?
3. What have you got up to in the question?
4. a tip for ii) to calculate theta, you should use two vectors that both 'emerge' from the same point, in this case point b
5. i worked it out eventually i knew it was wrong but couldnt work out why until my dad spotted that i reversed a sign. Thanks anyway
6. (Original post by katiealex99)
i worked it out eventually i knew it was wrong but couldnt work out why until my dad spotted that i reversed a sign. Thanks anyway
Great!

Just a tip for the future, we're all happy to provide help but we'd appreciate if you can post a pic of your working so far so we can guide you through the question - you'll understand it much better that way

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Updated: March 15, 2016
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