# What is the angle between <-9 , 2 , -4 >  and  < 5 , 2 , 0 > ?

Feb 16, 2016

139.25 deg.

#### Explanation:

Let the vectors be called AC and AB.
Then the vector BC = AB - AC = <14, 0, 4>.
The sides a, b and c of the$\triangle$ABC are 2sqrt53, sqrt101 and sqrt29.
The angle at A is arccos (( b.b + c.c - a.a ) / (2bc)).= arccos ( - 0..7576) =139.25 deg.