Both normal and inverse magnetoresistance (MR) response curves were observed for synthetic spin valves with the structure of Si(100)/Ru/Co(t(1))/Ru(0.7 nm)/Co(t(2))/Cu(3 nm)/Co(t(3))/Ru. Under the assumption of a coherent rotation of the magnetization in the three Co layers, the hysteresis loops, magnetization response, and MR response curves were calculated as a function of the parameters of the system. The parameters include antiferromagnetic coupling of Co(t(1)) and Co(t(2)) through Ru layer, a weak ferromagnetic coupling of Co(t(2)) and Co(t(3)) through Cu spacer, giant magnetoresistance of the Co/Cu/Co and Co/Ru/Co systems, and AMR contribution of each Co layer. The uniaxial anisotropy of each Co layer and a distribution of the coupling strength of Co(t(1))/Ru/Co(t(2)) were also included. To fit the experimental data well, it was necessary to include a biquadratic coupling in the Co(t(1))/Ru/Co(t(2)) trilayer. (C) 2001 American Institute of Physics.