Use vectors to find interior angles of the two triangles with given vertices - round to nearest hundredths

Question

Question 1: (-3, -4),(1,7),(8,2)

Question 2: (-3,5), (-1,9), (7,9)

Answer 1

all are done the same way, using the definition of the dot product:

u•v = |u|*|v|*cosθ

so, the angle between (-3, -4) and (1,7) can be figured:

-3-28 = -31 = 5*√50*cosθ
cosθ = -31/(5√50)
θ = 151.26°

proceed similarly with the others