The in-plane magnetic anisotropy and interlayer coupling have been investigated in the Fe/Si/Fe trilayers grown by de magnetron sputtering deposition on GaAs (001) substrates. The samples have been covered with thin Si cap layer. The measurements have been performed by means of ferromagnetic resonance (FMR) technique as a function of the orientation of dc magnetic field in plane of the film and at liquid helium and room temperatures. From these dependencies the in-plane cubic anisotropy constants as well as interlayer coupling constants are determined. It is shown that the uniaxial in-plane anisotropy has mainly magnetoelastic origin and directions of easy axes are different in both layers. Moreover, the anisotropy constants in both layers have different temperature dependencies related only to the differences in thermal expansion coefficients between Fe, Si and GaAs.