# Adding vectors to find displacement

• September 30th 2009, 10:24 AM
drkidd22
So I got this two vectors and I have used the pytagoream theorem to. But I'm still kinda sketchy about the answer I get.

46225+7225 = 53450
sqrt 53450 = 231km

Did I do it right?
• September 30th 2009, 12:47 PM
earboth
Quote:

Originally Posted by drkidd22
So I got this two vectors and I have used the pytagoream theorem to. But I'm still kinda sketchy about the answer I get.

46225+7225 = 53450
sqrt 53450 = 231km

Did I do it right?..........Unfortunately no

You only can use the Pythagorean theorem if the vectors are perpendicular to each other.

In your case the vectors include an angle of 135° and therefore you have to use the Cosine rule:

$d = \sqrt{85^2+215^2-2 \cdot 85 \cdot 215 \cdot \cos(135^\circ)}$

I've got $d \approx 281.59\ km$
• September 30th 2009, 02:11 PM
drkidd22
How did you find the angle. I don't seem to understand that part. But I do understand about the perpendicular stuff and the right triangle.