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Magnitude of vector CD?

  • Thread starter erpoi
  • Start date
  • #1
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C, D are points defined by position vectors c and d. Magnitude of c is 5, mag of d is 7, c dotproduct d is 4 ie c.d = 4, find the magnitude of vector CD.

So i started this way

c.d = magc*magdcos@
= 35cos@, @ = 83.4 degrees

But still no idea how do get magnitude of vector cd. Thank you!
 
Last edited:

Answers and Replies

  • #2
cristo
Staff Emeritus
Science Advisor
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Are you sure it isnt the magnitude of vector CD? (i.e. the vector from point C to point D)? If so, use the cosine rule on the triangle, since you know two vectors and an angle.
 
Last edited:
  • #3
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oops yes you are right, ok i use the cos rule and do get an answer(which is correct), but I am after it as an exact value (root66). How would i get that? Thanks!
 
Last edited:
  • #4
HallsofIvy
Science Advisor
Homework Helper
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?? You say you got an answer but you want an exact value? Did you use a calculator to get [itex]cos(\theta)[/itex]. Since you want to use [itex]cos(\theta)[/itex] in the cosine rule, why not just use [itex]cos(\theta)= \frac{4}{35}[/itex] rather than finding [itex]\theta[/itex] itself?
 

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