# Inequalities

## Half-planes

Recall that a non-vertical line is described by the equation $y=mx+b$. Notice that a straight line divides the plane into two parts. The best way to describe the two parts is by inequalities.

The set of point $(x,y)$ in the plane that satisfy the inequality $y < mx + b$ form the half plane "below" the line $y=mx+b$. While the set of points $(x,y)$ in the plane that satisfy the inequality $y > mx+b$ form the half-plane "above" the line $y=mx+b$.

Note that we use a dashed line to indicate that the points on the line are not included in half-planes. If we change the strict inequality $<$ to $\leq$, then we include the points on the line as well. We use a solid line to indicate that these points are included. Similarly, if we change $<$ to $\geq$ we then include the points on the line.

### Example. Solve the inequality $y < x$, and sketch the solution.

The set of points that satisfy this equation are those whose x-coordinate is larger than y-coordinate. Consider the line $y=x$. The inequality $<$ tells us that we want the points below the line $y=x$, and that agrees with having a larger x-coordinate than y-coordinate.

Let's try a more complicated example.

### Example. Solve the inequality $x-2y \leq -3x+2$, and sketch the solution.

First, let's rewrite the equation so that we can use what we know:
 $x-2y \leq -3x+2$ $-2y \leq -4x+2$ (Details) subtract $x$ from both sides $y \geq 2x-1$ (Details)divide both sides by $-2$, which switches the inequality
So we get the half-plane above the line $y=2x-1$, together with the line itself.