You can split things into parts.
You can share a snack fairly. This uses parts of a whole. It is like a piece of cake. These are called numbers. Can you find some parts today?
You can split things into parts.
These parts are called rational numbers. You can write them as a fraction. A fraction uses two whole numbers. One number goes on top. The other goes on the bottom. You can also use decimals. These decimals might end. Or they might repeat the same pattern.
Even whole numbers are rational numbers. They fit right into this group. Rational numbers are part of the real numbers.
Imagine you want to share a pizza fairly. You can cut it into equal parts. Each part is a number. We call these rational numbers.
A rational number is a fraction. It uses two whole numbers. One number is the numerator. It goes on top. The other is the denominator. It goes on the bottom. The bottom number cannot be zero. Every whole number is also a rational number. For example, the number 5 is a rational number. You can write it as 5 over 1.
You can also write these numbers as decimals. Some decimals end quickly. Others repeat the same pattern forever. If a number does not do this, it is called irrational. Most real numbers are actually irrational.
Rational numbers are very close to each other. You can always find another rational number between any two others. This makes them a dense set. You can also use them to do math. You can add, subtract, multiply, and divide them. This keeps them within the same group.
Have you ever thought about how we measure things that are not whole? If you have one cake and share it with two friends, you each get a piece. This piece is a part of a whole. In math, we call these parts rational numbers. A rational number is any number that can be written as a fraction. It uses two whole numbers called integers. The top number is the numerator. The bottom number is the denominator. The bottom number can never be zero.
These numbers work in many different ways. You can write them as fractions or as decimals. Some decimals are short and stop after a few digits. Other decimals go on forever but repeat the same pattern. For example, they might show the same sequence of digits over and over. Even if you use different bases, like binary, this rule stays the same. Every whole number is also a rational number. You can turn a number like 5 into a fraction by writing it as 5 over 1.
Math lovers have studied these numbers for a very long time. The word "rational" actually comes from a very old history. It was used to describe numbers as early as 1570. Interestingly, the word "ratio" came into use later, around 1660. Long ago, ancient Greek thinkers had a hard time with different kinds of numbers. They avoided certain lengths because they felt they were illogical. They called these lengths "irrational" because they were not easy to speak about.
There are many facts to know about how these numbers behave. You can add, subtract, multiply, and divide them easily. When you do these things, you always get another rational number. This makes them a special group called a field. Between any two rational numbers, you can always find another one. This means they are very crowded together on a number line. We call this being "dense." Even though they are everywhere, they are not the only numbers.
It is helpful to see how they fit into the bigger world of math. Rational numbers are part of a larger group called real numbers. Some real numbers are not rational at all. We call those irrational numbers. Examples include the square root of 2 or the golden ratio. In fact, almost all real numbers are actually irrational. The rational numbers are a countable set, which means we can list them in order. This helps us understand how they sit inside the huge world of real numbers.
A rational number is a specific type of value in mathematics. It is any number that can be expressed as a quotient of two integers. This means you can write it as a fraction. The top number is called the numerator. The bottom number is called the denominator. The denominator must be a non-zero integer.
Every whole number, or integer, is also a rational number. You can represent an integer like 5 as the fraction 5/1. This makes the set of rational numbers, often called "the rationals," a very broad group.
Rational numbers have specific decimal patterns. If you write a rational number as a decimal, it will do one of two things. First, the decimal might terminate, meaning it stops after a finite number of digits. For example, 1/4 becomes 0.25. Second, the decimal might repeat. It will eventually start a sequence of digits that repeats over and over. This rule works in base 10, but it also works in other bases like binary or hexadecimal. Numbers that do not follow these patterns are called irrational numbers. Examples of irrational numbers include the square root of 2 and the golden ratio.
In the study of arithmetic, rational numbers follow strict rules. You can add, subtract, multiply, and divide them. When you perform these operations on rational numbers, the result is always another rational number. Because of this, mathematicians call the rationals a "field." This is a special structure where these operations work consistently.
Every rational number also has an additive inverse, which is its opposite. For example, the opposite of 1/2 is -1/2. Every non-zero rational number has a multiplicative inverse, also known as a reciprocal. The reciprocal of 3/4 is 4/3.
There is a special way to write every rational number called canonical form. This is also known as an irreducible fraction. In this form, the numerator and denominator are coprime. This means they share no common factors other than 1. You can find this form by dividing both numbers by their greatest common divisor. For example, 6/8 can be simplified to 3/4. This simplified version is the unique canonical representative of that value.
The history of these numbers is quite surprising. The word "rational" was used to describe numbers as early as 1570. However, the word "ratio" did not appear with its modern meaning until about 1660. This means the term "rational" actually predates the word "ratio." The concept of "irrational" numbers has deep roots in ancient Greece. Some Greek thinkers found certain lengths to be illogical. They avoided thinking of these lengths as numbers. They called them irrational because they were "not to be spoken about."
Rational numbers have a unique property called density. This means that between any two rational numbers, there is always another rational number. You can keep finding new ones forever. Because of this, the rationals are a dense subset of the real numbers.
Despite being everywhere, the set of rational numbers is "countable." This means you can create a list that includes every single rational number. You can do this by sorting them by the sum of their numerator and denominator.
Even though they are countable, the set of real numbers is "uncountable." This implies that almost all real numbers are actually irrational.
Mathematically, the field of rational numbers is a "prime field." This means it is the smallest field that contains the integers. Any other field containing the integers must also contain the rationals. In mathematical analysis, the real numbers can actually be built using the rational numbers. This is done through processes called completion, using things like Cauchy sequences or Dedekind cuts. The rational numbers are a fundamental building block for the entire system of real numbers.
Where will your curiosity go next?
🖼️ Images & Media (3)
More to explore
✨ What else?
Related topics you might enjoy
🔬 Go deeper
More advanced topics to explore
🪜 Step back
Simpler topics to build understanding
What is Nepedia?
A free, ad-free encyclopedia for children. Every article is written at five reading levels, so the same page works for a five-year-old and a fifteen-year-old — use the level switcher above to see this one change. No account needed to read.