Question 72·Hard·Linear Inequalities in One or Two Variables
A point with both coordinates integers must satisfy the following system of inequalities:
How many such integer-coordinate points satisfy the system?
For linear inequality systems like this, avoid plotting every point. Instead, treat it algebraically: (1) find where the boundary lines intersect by setting them equal, (2) combine that with any given -range to list the integer -values that can work, and (3) for each such , compute the lower and upper bounds for and use the inclusive-count formula . This approach is fast, systematic, and reduces errors compared with trying to eyeball the graph.
Hints
Visualize or sketch the region
Think of the inequalities and as the regions above one line and below another. Also remember that is restricted between and .
Find where the boundary lines meet
Set equal to to find the -value where the two lines intersect. This tells you up to which -values the two inequalities can overlap.
List the integer x-values that can work
After combining the intersection information with , list all integer -values that actually allow some satisfying both inequalities.
Count integer y-values for each x
For each allowed integer , compute the lower and upper bounds for , then count how many integers lie between them (including endpoints) using .
Desmos Guide
Graph the two boundary lines as inequalities
In Desmos, type y >= 2x - 6 and y <= -x + 2 on separate lines so that the overlapping shaded region between the lines appears.
Add the x-range restriction
Either graph the vertical lines x = -1 and x = 5 as guides or restrict the inequalities using domain notation like y >= 2x - 6 { -1 <= x <= 5 } and y <= -x + 2 { -1 <= x <= 5 } so you only see the part of the region with .
Identify which integer x-values have overlap
On the graph, look at vertical slices at integer -values. You should see that only have a visible overlap (a vertical segment of shaded region) between the two inequalities.
Count integer y-values in each vertical slice
For each of those integer -values, note the -coordinates where the slice intersects the lower and upper boundaries, list the integer -values between them (inclusive), and then add these counts to find how many integer-coordinate points lie in the shaded region.
Step-by-step Explanation
Find where the two lines can overlap
We need points that satisfy both and .
First, find where the two boundary lines intersect by setting them equal:
So the lines intersect at , and for the two inequalities to have any common -values, must be at most , because beyond that the lower bound would be above the upper bound.
Determine which integer x-values are possible
We are also given the vertical restriction .
Combining this with , the actual -range where solutions can exist is
The integers in this interval are
For , the inequalities and cannot both be true at the same time, so there are no solutions for those -values.
For each integer x, find the y-range and count integer y-values
Now, for each allowed integer , find the range of .
Use "top minus bottom plus 1" to count how many integers lie between two integers :
- When :
- Lower bound:
- Upper bound:
- Integers from up to are allowed, so the count is
- When :
- Lower bound:
- Upper bound:
- Count:
- When :
- Lower bound:
- Upper bound:
- Count:
- When :
- Lower bound:
- Upper bound:
- Count:
Add the counts for all x-values
Add the number of integer -values for each allowed integer :
So there are integer-coordinate points that satisfy all three inequalities.