• Support PF! Buy your school textbooks, materials and every day products Here!

Finding the angle in between 3D vectors

  • Thread starter pugfug90
  • Start date
  • #1
118
0

Homework Statement


I did the rest of a problem right.. It asks to determine whether the parallelogram is a rectangle, meaning right angles..

A(2,-1,4), B(3,1,2), C(0,5,6), D(-1,3,8)




Homework Equations


cos theta = dot product of u and v divided by magnitude of u and v


The Attempt at a Solution


I drew a sketch parallelogram, with A at Northwest, B at NE, C at SE, and D at SW..
blah blah
I got vector AD and BC to be <-3,4,4> and vectors AB and DC to be <1,2,-2>

mag of AB/BC is square root 41, etc. if you can help me, you should know how I got there.
now..

cos theta = [(-3*1) +(4*2) + (4*-2)]/[square root of 41*square root of 9]

[-3]/[square root 369]=cos theta

theta=?98.9???

The back of my book says 81.02 degrees..
 

Answers and Replies

  • #2
529
1
You're not doing anything wrong. There are two different angles in a parallelogram which sum to 180 degrees. 98.9+81.1= 180.
 
  • #3
118
0
Thanks for that! :-D
 

Related Threads on Finding the angle in between 3D vectors

Replies
1
Views
3K
Replies
1
Views
2K
  • Last Post
Replies
2
Views
1K
Replies
4
Views
63K
  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
9
Views
2K
Replies
1
Views
774
  • Last Post
Replies
6
Views
1K
  • Last Post
Replies
1
Views
789
Top