M6MATH-8.3

Writing & Graphing Inequalities

Learn to write inequalities from real-world situations and graph their infinitely many solutions on a number line.

What you'll do in this lesson

A voice-first session with the Crimsora tutor on Writing & Graphing Inequalities, then targeted practice and FRQs — with the tutor adapting to where you get stuck.

What this lesson covers

Inequalities show us when something is not equal — when one amount must be greater than, less than, or at least as much as another. In real life, constraints like age limits for movie ratings, minimum balances for bank accounts, and speed limits all use inequalities. In this lesson, you will write inequalities to represent everyday situations and use a number line to show all the possible solutions.

Understanding Inequalities and Their Symbols

An inequality is a mathematical statement that compares two expressions using symbols other than equals. The four main inequality symbols are: greater than (>)(>), less than (<)(<), greater than or equal to ()(\geq), and less than or equal to ()(\leq).

When you see the phrase "more than" or "greater than," use >>. When you see "fewer than" or "less than," use <<. Phrases like "at least" and "no more than" are trickier. "At least" means the value can be that amount or more, so it uses \geq. "No more than" means it cannot go above that amount, so it uses \leq.

For example, if a swimming pool requires swimmers to be at least 10 years old, you would write a10a \geq 10, where aa is the age. If a bridge has a weight limit of 5,000 pounds, you write w5000w \leq 5000, where ww is the weight. Notice that an inequality has infinitely many solutions — not just one number, but an entire range of numbers that make the statement true.

Writing Inequalities from Real-World Situations

To write an inequality, identify the variable (what you're measuring), the number or constraint, and the relationship between them. Start by asking: Is the quantity greater, less, greater-or-equal, or less-or-equal to something?

Let's say a theme park requires visitors to be taller than 48 inches to ride the roller coaster. You'd write h>48h > 48, where hh is height in inches. Note that 48 is not included because the rule says "taller than," not "at least 48 inches."

If a restaurant needs at least 6 people to book a group reservation, you write p6p \geq 6, where pp is the number of people. Now 6 is included because "at least" means 6 or more.

A key skill is reading the situation carefully. Words like "more than," "fewer than," "at least," "no more than," "exceeds," and "not more than" each translate to a specific symbol. Practice matching the English phrase to its mathematical symbol, and always identify what the variable represents before you write the inequality.

Representing Inequalities on a Number Line

A number line graph shows all the solutions to an inequality at once. Start by drawing a horizontal line and marking the important number mentioned in the inequality.

If the inequality uses >> or << (strictly greater or less), draw an open circle at that number. The open circle means that exact number is not a solution. For example, h>48h > 48 gets an open circle at 48.

If the inequality uses \geq or \leq (greater-or-equal or less-or-equal), draw a filled circle at that number. The filled circle means that number IS a solution. For example, p6p \geq 6 gets a filled circle at 6.

Next, draw an arrow showing the direction of all solutions. For >> or \geq, the arrow points right (toward larger numbers). For << or \leq, the arrow points left (toward smaller numbers). The arrow represents the infinitely many solutions. For instance, h>48h > 48 has a number line with an open circle at 48 and an arrow extending right, showing that any height above 48 inches works. The arrow never ends because there is no upper limit.

Common Mistakes and How to Avoid Them

A frequent error is mixing up the inequality symbol. Students sometimes write h<48h < 48 when the problem says "taller than 48 inches," which is the opposite of what's needed. Always read the problem twice and point to the phrase that tells you which direction the inequality goes.

Another mistake is confusing when to use an open circle versus a filled circle. Remember: if the phrase includes the number itself — "at least," "no more than," "not less than" — use a filled circle. If it excludes the number — "more than," "fewer than," "greater than" — use an open circle.

Students sometimes graph the arrow in the wrong direction. Test yourself by picking a number on the arrow and checking: Does that number actually satisfy the inequality? If w5000w \leq 5000 and you pick 4000, does 400050004000 \leq 5000? Yes, so 4000 should be on your arrow. If you picked a number on your arrow and the inequality was false, your arrow is pointing the wrong way. Fix it by reversing direction.

Connecting Inequalities to Real Constraints

Inequalities model real limits in the world. A movie rated PG-13 requires viewers to be at least 13, so a13a \geq 13. A doctor's scale goes up to 350 pounds, so w350w \leq 350. A game requires more than 2 players, so p>2p > 2. Video game levels might unlock only after scoring at least 500 points, so s500s \geq 500.

When you solve a word problem, remember that your inequality should match the constraint exactly. If a rule says "no more than 5 items," the inequality n5n \leq 5 captures that. The number line then shows every possible value that obeys that rule. Understanding that an inequality describes a whole range of acceptable values — not just one answer — helps you see why infinitely many solutions makes sense. In a real classroom, many students might be at least 11 years old; the inequality a11a \geq 11 includes all of them at once.

Key terms

Inequality.
A mathematical statement that compares two expressions using symbols such as >>, <<, \geq, or \leq instead of an equals sign.
Greater than symbol (>>).
Shows that one quantity is strictly larger than another; the number itself is not included in the solution set.
Less than symbol (<<).
Shows that one quantity is strictly smaller than another; the number itself is not included in the solution set.
Greater than or equal to symbol (\geq).
Shows that one quantity is larger than or equal to another; the number itself is included in the solution set.
Less than or equal to symbol (\leq).
Shows that one quantity is smaller than or equal to another; the number itself is included in the solution set.
Open circle.
A hollow circle placed on a number line to show that a number is not included in the solution set of an inequality using >> or <<.
Filled circle.
A solid circle placed on a number line to show that a number is included in the solution set of an inequality using \geq or \leq.
Solution set.
All the values that make an inequality true; for most inequalities, this is an infinite set of numbers represented by an arrow on the number line.

Worked example

A community pool requires swimmers to be at least 8 years old. Write an inequality to represent this constraint, then graph the solution set on a number line.
Step 1: Identify the variable. Let aa represent the age of swimmers in years.

Step 2: Identify the constraint and translate it to an inequality symbol. The phrase "at least 8 years old" means the age must be 8 or greater, so we use the \geq symbol.

Step 3: Write the inequality. Combining the variable and the symbol with the number: a8a \geq 8.

Step 4: Graph on a number line. Draw a horizontal line and mark the number 8. Since the inequality is \geq (includes the number 8), draw a filled circle at 8. Since aa can be 8 or any age greater than 8, draw an arrow pointing to the right from the filled circle, extending indefinitely. This arrow shows that all values from 8 onward satisfy the constraint.

Your number line shows a filled circle at 8 and an arrow extending right. This represents all swimmers aged 8, 9, 10, 11, ... and beyond — infinitely many ages are allowed.

Practice questions

A concert venue must have no more than 500 people on the dance floor for safety reasons. Which inequality represents this constraint, where pp is the number of people?
  1. p>500p > 500
  2. p500p \geq 500
  3. p500p \leq 500
  4. p<500p < 500

Answer: p500p \leq 500

The phrase "no more than 500" means the number of people cannot exceed 500, so it can be 500 or fewer. This translates to p500p \leq 500. The \leq symbol includes 500 itself. If the phrase were "fewer than 500," you would use p<500p < 500 instead, but "no more than" is the same as "less than or equal to."
A smartphone app requires users to have a score of at least 100 to unlock the next level. Write an inequality for this situation using ss as the score, and describe what the graph of this inequality would look like on a number line.

Answer: s100s \geq 100. The number line would have a filled circle at 100 and an arrow pointing to the right, showing that scores of 100 and all scores above 100 are solutions.

"At least 100" means the score must be 100 or higher, so s100s \geq 100 is correct. The filled circle (not an open circle) is essential because 100 is included — a score of exactly 100 does unlock the level. The rightward arrow indicates that any score above 100 also works, and since scores could theoretically be infinitely high, the arrow extends without end. This represents infinitely many solutions on the number line.
A local library will only accept book donations of books in "good condition or better." If we measure condition on a scale from 1 to 10, with 10 being perfect, write an inequality using cc for condition and explain whether you should use an open or filled circle when graphing.

Answer: c7c \geq 7 (or another reasonable threshold like c8c \geq 8, depending on what "good" means on the scale). A filled circle should be used because "good condition or better" includes the boundary value.

This question requires you to interpret what "good condition" means numerically. On a 1–10 scale, you might reasonably say "good" starts at 7, 8, or even 6, depending on the library's standards. The key is recognizing that "or better" means the boundary value is included, so a filled circle is correct regardless of which number you choose. The phrase "or better" is another way of saying "or equal to," which always signals \geq or \leq rather than strict >> or <<.

FAQ

What is the difference between >> and \geq?
The symbol >> means strictly greater than — the number itself is not a solution. For example, x>5x > 5 means 5 is not included; only values like 5.1, 6, 7, etc., work. The symbol \geq means greater than or equal to — the number itself IS a solution. For example, x5x \geq 5 includes 5, along with 6, 7, and higher. On a number line, >> gets an open circle and \geq gets a filled circle.
Why does an inequality have infinitely many solutions?
An inequality describes a range or region of numbers, not just one specific answer. For example, x<10x < 10 is true for 9, 8.5, 8, 0, 3-3, and countless other numbers. You cannot list all of them because there are always more numbers between any two points on the number line. That is why we use an arrow on the number line to show the entire infinite set at once.
How do I know which direction to point the arrow on a number line?
The arrow points in the direction of the solutions. For >> or \geq, the arrow points right (toward larger numbers). For << or \leq, the arrow points left (toward smaller numbers). A quick check: pick any number on your arrow and substitute it into the inequality. If the inequality is true, your arrow is correct. If it is false, flip the arrow direction.
Does an inequality always include the number mentioned in the problem?
No. If the problem uses words like "more than," "greater than," "fewer than," or "less than," the number is NOT included, so use an open circle and >> or <<. If the problem uses words like "at least," "no more than," "no less than," or "not more than," the number IS included, so use a filled circle and \geq or \leq. Reading the exact wording is critical.

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