Question 47·Medium·Linear Inequalities in One or Two Variables
Consider the system of inequalities
Which of the following points is a solution to the system?
For multiple-choice questions asking which point satisfies a system of inequalities, the fastest SAT method is usually to plug in the answer choices. For each choice, substitute the coordinates into each inequality and see if the resulting statements are true, being careful with strict () versus non-strict () inequalities. Eliminate any choice that fails even one inequality, and the remaining point that makes all inequalities true is your answer; this avoids the extra time of fully graphing the system unless the question specifically benefits from a visual approach.
Hints
Recall what a solution to a system is
A point is a solution to a system of inequalities only if it makes every inequality in the system true at the same time.
Plug in coordinates for x and y
For each answer choice, use its coordinates as and in the inequalities. Compute and and check them against and with the correct inequality symbols.
Pay attention to the inequality symbols
In the first inequality, the symbol is (strictly less than), so equality does not count. In the second, the symbol is (less than or equal to), so equality does count.
Desmos Guide
Graph the inequalities
In Desmos, enter the two inequalities as separate expressions:
x - 2y < 43x + y <= 5Desmos will draw the boundary lines and shade the regions that satisfy each inequality.
Identify the solution region
Look for the region where the shadings from both inequalities overlap. That overlapping area represents all points that satisfy both inequalities at the same time.
Plot the answer choices as points
Add each choice as a separate point in Desmos by typing (4,0), (2,2), (0,4), and (1,3) as four different expressions. Check visually which of these plotted points lies inside the overlapping shaded region from step 2; that point is the solution to the system.
Step-by-step Explanation
Understand what a solution to a system means
A point is a solution to the system if, when you plug and into both inequalities, each inequality becomes a true statement.
So here we need a point that makes both and true.
Use substitution to test the answer choices
For each choice, treat the coordinates as and and substitute:
- In the first inequality, compute and check if it is less than .
- In the second inequality, compute and check if it is less than or equal to .
A point is a solution only if it passes both checks.
Eliminate choices that fail at least one inequality
Test the first, second, and fourth choices:
-
Choice A, :
- First inequality: . We need , but is not less than , so this fails.
- It also fails the second inequality: , and is false.
-
Choice B, :
- First inequality: , and is true.
- Second inequality: , and is false, so this point is not a solution.
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Choice D, :
- First inequality: , and is true.
- Second inequality: , and is false, so this point is not a solution.
So A, B, and D are all eliminated because each one fails at least one inequality.
Check the remaining choice and conclude
Now test choice C, :
- First inequality: , and is true.
- Second inequality: , and is true.
Since makes both inequalities true, it is the only point that is a solution to the system. The correct answer is .