Answer:

The second question:

Answer:

The third question:

Answer:

This part of the extended materials mainly examines the knowledge points of inequality groups:

To solve inequality groups, we can first calculate their solution sets one by one, and then express them on the number axis respectively. Take the inequality group composed of two inequalities as an example:

(1) If the solution sets of two unknowns are the same to the left on the number axis, the solution set of the unknown on the left is taken as the solution set of the inequality group, which is "taking the smallest as the largest".

(2) If the solution sets of two unknowns are in the same direction to the right on the number axis, the solution set of the unknown on the right is taken as the solution set of the inequality group, which is "taking the largest as the largest".

③ If the solution sets of two unknowns intersect on the number axis, the value between them is taken as the solution set of the inequality group. If x represents the solution set of inequality, it is generally expressed as a.

(4) If the solution sets of the two unknowns are backward on the number axis, the solution set of the inequality group is empty and the inequality group has no solution. This is "empty from the back".