# Magnitude of vector CD?

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:

cristo
Staff Emeritus
?? You say you got an answer but you want an exact value? Did you use a calculator to get $cos(\theta)$. Since you want to use $cos(\theta)$ in the cosine rule, why not just use $cos(\theta)= \frac{4}{35}$ rather than finding $\theta$ itself?