The angle between the planes is the same as the angle between their normal vectors, which are
n₁ = ⟨1, 1, 1⟩
n₂ = ⟨4, 3, 1⟩
The angle θ between the vectors is such that
⟨1, 1, 1⟩ • ⟨4, 3, 1⟩ = ||⟨1, 1, 1⟩|| ||⟨4, 3, 1⟩|| cos(θ)
Solve for cos(θ) :
4 + 3 + 1 = √(1² + 1² + 1²) √(4² + 3² + 1²) cos(θ)
8 = √3 √26 cos(θ)
cos(θ) = 8/√78
