Further Vectors help (1 Viewer)

Tantrrsh

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The angle between two vectors u=i-3j and v=ai+5j is 120°. Find a correct to 1 decimal place.
I've tried substituting into the scalar equation but I can't get the answer
 

Trebla

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What do you end up with in your working?
 

InteGrand

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The angle between two vectors u=i-3j and v=ai+5j is 120°. Find a correct to 1 decimal place.
I've tried substituting into the scalar equation but I can't get the answer
 
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Tantrrsh

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I'm also stuck on this one:
Find vector b given a = 3i - j, a•b = -6 , and the angle between a and b is 30°.
 

InteGrand

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I worked it out, and my answers are 24.4 and -4.4. However, the text book says 4.4 or -24.4. Why is that?
You should probably show us the steps in your working, in order for us to be able to answer that.
 

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