# Divisibility Rule for 11: A Simple Hack to Save Time on Math Tests!

As we learn basic mathematics, divisibility tests are one of the first things we learn. Divisibility tests are tricks or rules that can be applied to a number to check if it’s divisible by another number without actually performing the division operation. Divisibility rules are useful in many areas of mathematics and everyday life.

One of the most commonly used and easily applicable divisibility tests is the rule for divisibility by 11. In this article, we’ll explore the logic behind the rule for divisibility by 11, its applications, and some interesting facts about it.

## Understanding the Rule for Divisibility by 11

The rule for divisibility by 11 is simple: If the alternating sum of the digits of a number is divisible by 11, then the number itself is also divisible by 11. Let’s break down this rule to understand it better.

First, let’s consider a number, say 12345. To apply the rule, we’ll take the alternating sum of its digits. That means we’ll take the sum of the digits in the odd places (1, 3, and 5) and subtract the sum of the digits in the even places (2 and 4). In this case, the sum of the odd-place digits is 1+3+5=9, and the sum of the even-place digits is 2+4=6. Therefore, the alternating sum of the digits of 12345 is 9-6=3.

Now, we need to check if this alternating sum is divisible by 11. In this case, since 3 is not divisible by 11, we can say that 12345 is not divisible by 11.

Let’s take another example, say 286. The alternating sum of its digits is 2-8+6=0, which is divisible by 11. Therefore, we can conclude that 286 is also divisible by 11.

The rule for divisibility by 11 works for any number, no matter how many digits it has.

## Applications of the Rule for Divisibility by 11

The rule for divisibility by 11 has many practical applications. It can be used to quickly check if a given number is divisible by 11 or not. For example, if you’re working with a large number and need to check if it’s divisible by 11, you can apply the rule without actually performing the division operation.

The rule for divisibility by 11 is also useful in cryptography, where it’s used to generate hash codes for security purposes. Hash codes are generated by applying a mathematical function to a message or data to create a fixed-size output. The rule for divisibility by 11 is used in some hash functions to ensure that the output is a multiple of 11, which makes it easier to verify the integrity of the data.

## Fun Facts about the Rule for Divisibility by 11

• The rule for divisibility by 11 is sometimes called “the elevator rule.” This is because the rule can be demonstrated by looking at the numbers on an elevator button panel. If you add up the numbers of any two adjacent buttons, the sum is always a multiple of 11.
• The rule for divisibility by 11 can also be used to check if a number is prime or not. This is because if a number is not divisible by any number less than its square root, then it’s prime. If the alternating sum of its digits is not divisible by 11, then it’s not divisible by 11, which means it’s also not divisible by any of its factors.
• The rule for divisibility by 11 can also be extended to negative numbers. To apply the rule to a negative number, simply ignore the negative sign and apply the rule to the absolute value of the number.
• The rule for divisibility by 11 is a special case of a more general rule called the “casting out nines” rule. The casting out nines rule can be used to check if a number is divisible by 9 or not.
• The rule for divisibility by 11 has been known for centuries and was first described in the works of the ancient Indian mathematicians. It was later popularized by the Persian mathematician Al-Khwarizmi in the 9th century.

## Advantages of Using the Rule for Divisibility by 11

The rule for divisibility by 11 has many advantages over traditional division methods. Some of the advantages include:

• Speed: The rule for divisibility by 11 is much faster than performing long division, especially for large numbers.
• Mental Math: The rule for divisibility by 11 is an excellent tool for mental math, as it only requires basic arithmetic skills.
• Error Checking: The rule for divisibility by 11 is an excellent way to check if the result of a division operation is correct.
• Cryptography: The rule for divisibility by 11 is used in some cryptographic applications, making it an essential tool for data security.

## Conclusion

The rule for divisibility by 11 is a simple but powerful tool that can be used to quickly check if a number is divisible by 11 or not. The logic behind the rule is easy to understand, and it has many practical applications in everyday life and mathematics. By applying the rule, we can perform calculations quickly and accurately, saving time and reducing the risk of errors.

## FAQs

### Can the rule for divisibility by 11 be extended to other numbers?

Yes, there are divisibility rules for other numbers as well, such as 2, 3, 4, 5, 6, 8, 9, and 10.

### How is the rule for divisibility by 11 used in cryptography?

The rule for divisibility by 11 is used in some hash functions to ensure that the output is a multiple of 11, which makes it easier to verify the integrity of the data.

### Is the rule for divisibility by 11 only applicable to positive numbers?

No, the rule for divisibility by 11 can be extended to negative numbers as well.

### Can the rule for divisibility by 11 be used to check if a number is prime or not?

Yes, the rule for divisibility by 11 can be used to check if a number is not divisible by any of its factors, including 1 and itself.

### Who first discovered the rule for divisibility by 11?

The rule for divisibility by 11 was first described in the works of the ancient Indian mathematicians and later popularized by the Persian mathematician Al-Khwarizmi in the 9th century.