Question 35·Medium·Linear Inequalities in One or Two Variables
In the -plane, the point satisfies the system of inequalities
Which of the following could be the value of ?
For SAT questions asking which value makes a point satisfy a system of inequalities, first substitute the point’s coordinates into each inequality so everything is in terms of the unknown (here, ). Solve each inequality carefully, paying attention to the inequality direction, then combine the results into a single range. Finally, quickly test the answer choices against that range instead of plugging each one back into both original inequalities.
Hints
Use the coordinates of the point
The point gives you and . Replace and with these in both inequalities.
Turn the system into inequalities in a
After substituting, you should get two inequalities that each involve only . Solve each one step by step (add or subtract first, then divide if needed).
Remember both inequalities must be true
The correct must satisfy both inequalities at the same time. After you find the range of that works, check which answer choices fall inside that range.
Desmos Guide
Graph the boundary lines and the given y-value
In Desmos, enter the three equations:
y = 4x - 10y = x + 7y = 5
These are the two boundary lines from the inequalities and the horizontal line that corresponds to for the point .
Find where y = 5 meets each boundary line
Use Desmos’s intersection tool (tap where the lines meet) to find the intersection of y = 5 with y = 4x - 10, and also with y = x + 7. Note the x-coordinates of these two intersection points; those x-values are the endpoints of the interval of -values that make equal to each boundary.
Determine which x-values make the inequalities true
On the graph, look along the horizontal line y = 5 between those two intersection x-values. In this region, y = 5 is above the line y = 4x - 10 and below the line y = x + 7, matching the inequalities and . Check which answer choices have x-values lying between those two intersection x-values.
Step-by-step Explanation
Substitute the point into the inequalities
The point means and .
Substitute these into both inequalities:
- From you get
- From you get
Now you have two inequalities involving only .
Solve the first inequality for a
Solve step by step:
Divide both sides by :
This is the same as (about ).
Solve the second inequality for a
Now solve :
This tells you must be greater than .
Combine the conditions and check the choices
Both inequalities must be true at the same time, so has to satisfy both:
Together, this gives the interval
Now check each answer choice:
- -3 is not greater than .
- -1 is between and .
- 4 and 6 are not less than .
So the only possible value of from the choices is .