k6 - x1*(k3 + k5) + k7*x4 + k7*x5 - k1*k2*x1*x2 
k9 - x2*(k3 + k5) + k7*x4 + k7*x6 - k1*k2*x1*x2 
k1*k2*x1*x2 - x3*(k3 + k4 + k5) 
k5*x3 - k7*x4 
k5*x1 - k7*x5 
k5*x2 - k7*x6 
