Given planes are x + y – 2z = 3 and 2x – 2y + z = 5

We know that angle between two planes,

a₁x + b₁y + c₁z + d₁ = 0

a₂x + b₂y + c₂z + d₂ = 0 is given as

Here we have,

a₁ = 1, b₁ = 1, c₁ = – 2

a₂ = 2, b₂ = – 2, c₂ = 1

Hence, the angle between planes x + y – 2z = 3 and 2x – 2y + z = 5 is