Question 77·Medium·Linear Inequalities in One or Two Variables
For which of the following tables are all the listed pairs solutions to the system of inequalities?
For systems of inequalities with answer choices given as tables of points, do not graph unless necessary—plug in. For each table, quickly test each listed pair: compute to check it is at least , and compute to check it is strictly less than . The moment a single pair in a table fails either inequality, cross out that table and move on; this process of elimination is much faster and less error-prone than trying to reason visually or guessing.
Hints
Think about the definition of a solution
For a pair to be a solution to the system, what must happen when you plug it into each inequality?
Test pairs systematically
Pick one table and test its pairs one by one: first compute and compare it to , then compute and compare it to .
Watch the inequality symbols
Pay special attention to the difference between and : one allows equality, the other does not. How does that affect pairs where equals ?
Eliminate, don’t guess
If even one pair in a table fails either inequality, you can cross out that entire table and move on to the next.
Desmos Guide
Use Desmos to evaluate for each pair
For each pair in a table, type an expression like 3*(x_value) - (y_value) (for example, 3*2 - 2 for ). Compare the numerical result Desmos gives you to and decide whether it is . If any pair gives a result less than , eliminate that table.
Use Desmos to evaluate for each pair
For the same pairs, type (x_value) + (y_value) (for example, 2 + 2 for ). Compare the result to and check whether it is strictly less than . If any pair gives or more, eliminate that table.
Identify the correct table
After checking all three pairs in each table with these two expressions, the correct table is the one where every pair produces a value for and a value for when evaluated in Desmos.
Step-by-step Explanation
Understand what it means to be a solution
A pair is a solution to the system if both inequalities are true when you substitute and :
- Check .
- Check .
If a pair fails either inequality, it is not a solution. A table is correct only if all three of its pairs work for both inequalities.
Use the first inequality to eliminate tables
Start by checking for the pairs in each table.
Example with a pair from one table:
- For : , and is not , so does not satisfy the first inequality. Any table containing cannot be correct.
Do a similar quick check for at least one pair in each table; if a single pair fails , you can discard that whole table.
Use the second inequality to catch strict-inequality failures
For remaining candidate tables, check the second inequality .
Be careful: means strictly less than, so is not allowed.
- For example, if you find a pair where , then that pair fails the second inequality, and its entire table cannot be correct.
After this step, only tables whose pairs all pass both checks should remain.
Verify all pairs in the remaining table(s)
Take the table that has not been eliminated and verify each of its three pairs carefully:
- For : and .
- For : and .
- For : and .
All three pairs satisfy both inequalities, so the correct table is the one listing , , and .