The aim of this paper is to study the symmetry properties of solutions of bi-harmonic differential equations of the type Δ𝟐+||-| 𝜶 𝒖 = 𝟎 and Δ𝟐 𝒖 + 𝒇(𝒖) = 𝟎 𝒊𝒏 𝛀 ⊂ 𝑹𝟐 We employ the method of moving planes, which is based on the Maximum principles in...