Vectors. Triangle Angle between vector

[SpringWater]

Homework Statement

Consider the points A(1,2) B(-2,-1) and C(3,-2) on the Cartesian plane.

1. Find the length of the segments AB and BC

2. Find the angle of the triangle at the vertex B

Homework Equations

Cosθ=(AB*BC)/|AB| |BC| ?

The Attempt at a Solution

The length of the segments; AB= <-3,-3> |AB|=√(18)
BC =<5,-1> |BC|=√(26)

So therefore the angle at point B is θ=arccos(((-3)(5)) (dot) ((-1)(-3))) / (√(18*26))

θ=arccos(-18 / √(468))

is this correct?

I am assuming that the dot product would be the (x1)(x2) + (y1)(y2)

 

[Dick]

The idea looks right, but (-3)(5)+(-1)(-3) isn't -18.