34Angle between two vectors

Find the angle between ⟨1, 2⟩ and ⟨3, −1⟩ in degrees. Give the answer to 3 decimal places.

The angle between ⟨1, 2⟩ and ⟨3, −1⟩ is about 81.870°. Using cosθ = (u·v)/(|u||v|), the dot product is 1, the lengths are √5 and √10, so cosθ = 1/√50 and θ = cos⁻¹(1/√50) ≈ 81.870°.