(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

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)

2. Relevant equations

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

3. 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..

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Finding the angle in between 3D vectors

**Physics Forums | Science Articles, Homework Help, Discussion**