Question 120·Hard·Linear Inequalities in One or Two Variables
Integers and must satisfy the system of inequalities
and also lie in the window and .
How many ordered pairs satisfy all of these conditions?
For SAT questions asking for the number of integer solutions to a system of inequalities in a bounded window, avoid brute-force guessing or drawing an imprecise picture. Instead, algebraically rewrite each inequality to express (or ) in terms of the other variable, then use substitution to find the exact integer range for one variable. For each integer in that range, find the corresponding inclusive range of the other variable and count how many integers fit; finally, sum those counts. Pay close attention to strict (, ) versus inclusive (, ) inequalities and to the given window bounds so you do not accidentally add or miss boundary points.
Hints
Rewrite the second inequality
Focus on . How can you rewrite this inequality to show in terms of ? Remember that and are integers, so "greater than 1" means at least how much bigger?
Trap x between a lower and upper bound
From the two inequalities, you can write one inequality that says must be at least something (in terms of ) and another that says must be at most something (in terms of ). What are those two expressions? Don't forget the window .
Limit the possible y-values
Use the lower bound on you found (in terms of ) and plug it into to see which values are even possible. Once you know the integer range for , you can systematically test each one and count how many integer values work.
Count systematically by rows
For each allowed integer , find the smallest and largest integer that satisfy all inequalities and the window. Count how many values are in that range (inclusive) for each , then add all those counts together.
Desmos Guide
Graph the boundary lines and inequalities
In Desmos, enter the two inequalities:
2x + 3y <= 18x - y > 1
Desmos will shade the region that satisfies both.
Add the window and focus on the correct region
Add the boundary lines for the window:
x = -3,x = 8y = -3,y = 8
Adjust the graph so you can clearly see the rectangle from to and to . The valid region is the overlap of the inequality shading inside this rectangle.
Count the integer lattice points
Turn on the grid (if it is not already), and zoom so that each integer grid point (where grid lines cross) is clearly visible. Count all intersection points with integer coordinates inside the rectangle that lie within the shaded overlap of the inequalities. The number of such grid points is the answer.
Step-by-step Explanation
Rewrite the inequalities in terms of x
We are given
and , with and integers.
From and the fact that and are integers, we get
- means is at least 2 more than .
- So .
From , solve for :
So for any fixed integer , must satisfy
- (from ),
- (from ),
- and (from the window).
Find which y-values can actually work
We need to exist that meets all constraints for a given .
Use the smallest possible from to test if the inequality can ever be true.
Take and plug into :
Since is an integer, this gives .
We also have the window , so combining:
- .
Only the integer values can possibly give solutions.
For each allowed y, find the x-range and count solutions
For each integer from to , must satisfy all of:
- ,
- ,
- .
So the actual range for each is
- lower bound: ,
- upper bound: .
Now compute for each :
-
:
- , so .
- , so .
- values: → solutions.
-
:
- , so .
- , so .
- values: → solutions.
-
:
- , so .
- , so .
- values: → solutions.
-
:
- , so .
- , so .
- values: → solutions.
-
:
- , so .
- , so .
- values: → solutions.
-
:
- , so .
- , so .
- values: → solutions.
Add up all the solutions
Now total the number of integer pairs from each -row:
- For :
- For :
- For :
- For :
- For :
- For :
Add them:
So there are ordered integer pairs that satisfy all the given conditions, which matches answer choice C.