Form 4 · Chapter 6

Systems of Linear Inequalities in Two Variables

Find the feasible region of a system of inequalities and check which points satisfy every constraint.

A system of linear inequalities is two or more inequalities that must hold at the same time. This subtopic covers systems, the feasible region, and testing whether a point satisfies all constraints.

What is a system?

A system of linear inequalities in two variables is a set of inequalities in $x$ and $y$ considered together. A point is a solution only if it satisfies every inequality in the system.

Example

For the system $y \geq 0$, $x \geq 0$, $x+y \leq 4$, the point $(1,2)$ works: $2\geq0$, $1\geq0$ and $1+2=3\leq4$ are all true.

Feasible region

The feasible region is the set of all points satisfying all the inequalities — the overlap of the individual shaded regions. Shade each inequality, and the common area is the feasible region.

Key formula

$\text{Feasible region} = R_1 \cap R_2 \cap \cdots \cap R_n$

Checking a point

Substitute the point into each inequality. If it fails even one, it is not in the feasible region. If no points satisfy all constraints, the region is empty (no solution).

Remember

All constraints must hold at once. The feasible region is the intersection (overlap) of every shaded region.

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