Question 21·Hard·Linear Inequalities in One or Two Variables
Which of the following tables lists three points, each of which lies in the solution set of the system of inequalities
For SAT questions asking which points satisfy a system of inequalities, it is usually fastest to plug each point directly into the inequalities instead of graphing. For each table, test one point at a time: compute and compare it to , then compute and compare it to . The moment you find a point in a table that makes either inequality false, eliminate that entire answer choice and move on; the correct table is the only one where every listed point makes both inequalities true.
Hints
Focus on the meaning of a solution
A point is a solution of a system of inequalities only if it makes every inequality in the system true. How can you test that for a specific pair?
Try plugging in coordinates
Pick one point from any table and substitute its and values into and then into . Decide if each resulting statement is true or false.
Use elimination efficiently
If any one point in a table makes either inequality false, you can eliminate that whole answer choice. You are looking for a table where all three points pass both tests.
Desmos Guide
Graph the system of inequalities
In Desmos, enter the two inequalities exactly as they are: 2x - 3y <= 6 and on the next line x + y > 4. You should see two shaded regions; the solution set of the system is the overlapping (darker) shaded region.
Plot the points from each answer choice
For a given answer choice, create a table in Desmos with columns x1 and y1, and enter the three pairs from that table as the rows. Desmos will plot these three points on the same coordinate plane as the inequalities.
Determine which table fits the solution set
Check, for each answer choice, whether all three plotted points lie inside the overlapping shaded region of the two inequalities. The correct answer is the choice whose three points are all in that intersection region and none are outside it.
Step-by-step Explanation
Understand what the system is asking
The system of inequalities is
A point is a solution to this system only if both inequalities are true when you plug in and . For the correct table, all three points listed must satisfy both inequalities.
Practice checking a single point
Take a sample point, such as , and plug it into each inequality:
- First inequality :
- , and is true.
- Second inequality :
- , and is true.
So is in the solution set. You will use this same plug-in process to test every point in each answer choice.
Eliminate tables that have any point failing an inequality
Now test the points in each of the first three answer choices. You only need one failure in a table to eliminate that choice.
-
Choice A points: , ,
- in : , and is false, so this table is not correct.
-
Choice B points: , ,
- in : , and is false, so this table is not correct.
-
Choice C points: , ,
- in : , and is false, so this table is not correct.
All three of these choices are eliminated because each contains at least one point that fails the first inequality.
Verify the remaining table and state the answer
The only remaining option is choice D, with points , , and . Check each one:
- :
- , and is true;
- , and is true.
- :
- , and is true;
- , and is true.
- :
- , and is true;
- , and is true.
All three points in this table satisfy both inequalities, so the correct answer is the table with , , and (choice D).