x_1, x_2, x_3, x_4, x_5, x_6, x_7, x_8, x_9, 
x_6
x_9
x_4+3*x_1
x_1*x_5+x_3
2*x_1*x_7^2+x_2
-2*x_2*x_3*x_7+x_1*x_7+x_1*x_8
