n = 11; x = { x1, x2, x3, x4, x5, x6, x7, x8, x9, x10, x11 }; dx = Simplify[ x[[ Range[2,n] ]] - x[[ Range[1,n-1] ]] ]; ddx = Simplify[dx[[ Range[2,n-1] ]] - dx[[ Range[1,n-2] ]] ]; m = 4; ddz = ddx; ddz[[m-2]] = a; ddz[[m-1]] = b; dz = dx; Do[ dz[[i]]=dz[[i-1]]+ddz[[i-1]], {i,2,n-1} ]; dz = Simplify[dz]; z=x; Do[ z[[i]]= z[[i-1]]+ dz[[i-1]], {i,2,n} ]; z = Simplify[ z]; eq1 = z[[m]] == x[[m]] - H; eq2 = z[[m+1]] == x[[m+1]] - H; sol = Simplify[ Solve[ {eq1,eq2}, {a,H} ]]; a -> (x2 - x3 - x4 + x5)/2, H -> (x2 - 3*x3 + 3*x4 - x5)/2 This is only the best approximation but it is not necesserily accurate!