# Thread: Angles between vectors (i, j, k)

1. ## Angles between vectors (i, j, k)

find the angles of the triangle formed by the points P(2,-2,5), Q(2,1,7) and R(7,6,-10)

do i use the dot product for this? which pairs would i use? does anyone know how to do this?

I got the following angles, are these right?

between P & Q = 28.8 degrees
" Q & R = 120 degrees
" P & R = 127.9 degrees

2. That is not the way to do it.

$\displaystyle m(\angle QPR) = \arccos \left( {\dfrac{{\overrightarrow {PQ} \cdot \overrightarrow {PR} }}{{\left\| {\overrightarrow {PQ} } \right\|\left\| {\overrightarrow {PR} } \right\|}}} \right)$

3. i did you way and got all my angles adding upto 195 and i think it should be 180. the angles i got where:

QPR = 95.4
QRP = 27.3
RQP = 73.4

I must be really dumb lol

4. Your $\displaystyle m(\angle QRP)$ is incorrect.
Try again. It is about 11.242deg.