1. The line becomes a step.

2. Take a sub-formula that is not the highest order as the main element.

3. The rest are independent variables.

Starting from the bottom of the ladder, this question comes to a conclusion: x4=2 has x3=- 1, then x2=0, and then x 1= 1.

Methods primary school calculation method

1 forcibly solved the relationship of x 1, x2, x3, x4.

Take a variable and find a general solution.