Question

Find the cosine of the angle between the planes x+y+z=0 and 4x+3y+z=1.

Answer: cosθ=

Answer 1

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