# How do you use vectors to find the interior angles (in radians) of a triangle with the vertices (1,2) (3,4) and (2,5)?

Let the vertices be A(1,2), B(3,4), & C(2,5).
Then vector$\vec{A B}$$= \left(2 , 2\right) ,$ since,
vector$\vec{A B}$=(position vector of $B$) - (position vector of $A$)
$= \left(3 , 4\right) - \left(1 , 2\right) = \left(3 - 1 , 4 - 2\right) = \left(2 , 2\right)$
Similarly, vector $\vec{B C} = \left(- 1 , 1\right) ,$ & vector $\vec{C A} = \left(- 1 , - 3\right) .$