Question

Difficulty: HardSystems of Linear Inequalities in Two Variables

A bounded region in the xyxy-plane is defined by the following system of inequalities:

y12x+2y2x1yx+11\begin{aligned} y &\ge \frac{1}{2}x + 2 \\ y &\le 2x - 1 \\ y &\le -x + 11 \end{aligned}

What is the maximum yy-coordinate of any point within this region?

Answer: 7

Answer

The maximum y-coordinate of any point in the bounded region is 7.
The solution region is a triangle with vertices at (2,3)(2, 3), (4,7)(4, 7), and (6,5)(6, 5). Since all inequalities are less-than-or-equal-to or greater-than-or-equal-to, the boundary points are included in the solution set. The maximum y-value occurs at the vertex (4,7)(4, 7), which gives a maximum y-coordinate of 7.

Step-by-Step Solution

1
Determine the equations of the boundary lines.
The boundary equations are y=12x+2y = \frac{1}{2}x + 2, y=2x1y = 2x - 1, and y=x+11y = -x + 11.
These equations represent the boundaries of the system of inequalities.
2
Find the intersection point of y=2x1y = 2x - 1 and y=12x+2y = \frac{1}{2}x + 2.
Solving 2x1=12x+22x - 1 = \frac{1}{2}x + 2 yields x=2x = 2, which gives y=3y = 3. The intersection is (2,3)(2, 3).
This is one vertex of the bounded region.
3
Find the intersection point of y=2x1y = 2x - 1 and y=x+11y = -x + 11.
Solving 2x1=x+112x - 1 = -x + 11 yields x=4x = 4, which gives y=7y = 7. The intersection is (4,7)(4, 7).
This is the second vertex of the bounded region.
4
Find the intersection point of y=x+11y = -x + 11 and y=12x+2y = \frac{1}{2}x + 2.
Solving x+11=12x+2-x + 11 = \frac{1}{2}x + 2 yields x=6x = 6, which gives y=5y = 5. The intersection is (6,5)(6, 5).
This is the third vertex of the bounded region.
5
Compare the y-coordinates of the three vertices of the bounded region.
The y-coordinates of the vertices (2,3)(2, 3), (4,7)(4, 7), and (6,5)(6, 5) are 3, 7, and 5, respectively. The maximum value is 7.
The maximum value of a linear coordinate over a bounded convex region occurs at one of its vertices.

Key Concept

Finding the vertices of a bounded region defined by a system of linear inequalities and optimizing a coordinate value.
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