# Algebraic Divisibility Hack: Divide by 3 Easily

**Understanding Divisibility by 3**

Divisibility by \( 3 \) is a fundamental concept in number theory. A number can be evenly divided by \(3\) without leaving a remainder. The algebraic method effectively proves this divisibility, allowing us to determine whether a given number is divisible by \(3\) or not.

## Rule 1: The number is divisible by \(3\) if the sum of digits is a multiple of \(3\).

**A Sum of Digits**: First, take any positive integer and find the sum of its digits. For example, if you have \(342\), you would add its digits together: \(3 + 4 + 2 = 9\).**Check for Multiples of \(3\)**: Next, check whether the sum you calculated in step \(1\) is itself a multiple of \(3\). In our example, the sum is \(9\), and \(9\) is a multiple of \(3\) because \(3\) times \(3\) equals \(9\).**Result**: If the sum of the digits is a multiple of \(3\), then the original number is divisible by \(3\). In our example, since \(9\) is a multiple of \(3\), the number \(342\) is divisible by \(3\).

In a nutshell, the rule tells us that if the digits of a number can be added up to a multiple of \(3\) (such as \(3, 6, 9, 12, 15\), etc.), then the number itself is divisible by \(3\). If the sum of the digits doesn’t result in a multiple of \(3\), then the number is not divisible by \(3\).

Let’s take a few more examples to illustrate:

Number: \(417\)

- Sum of digits: \(4 + 1 + 7 = 12 \)
- \(12\) is a multiple of \(3\) (\(12 \div 3 = 4\))
- Result: \(417\) is divisible by \(3\).

Number: \(728\)

- Sum of digits: \(7 + 2 + 8 = 17\)
- \(17\) is not a multiple of \(3\) (\(17 \div 3 = 5\) with a remainder)
- Result: \(728\) is not divisible by \(3\).

This rule is particularly helpful for mental math and quickly identifying whether a number can be evenly divided by \(3\) without requiring long division. It’s a basic but valuable tool in elementary mathematics.

## Rule 2: The product of three consecutive integers is a multiple of \(3\).

### Example

(a) Prove the product of three consecutive integers is divisible by \( 3 \).

\( \begin{align} &\text{Consider three consecutive positive integers are } n,n+1,n+2 \text{ where } n \text{ is a positive integer} \\ &\text{So the product of three consecutive positive integers: } n(n+1)(n+2) \\ &n \text{ can be in the form of } 3k, 3k+1 \text{ or } 3k+2 \text{ where } k \text{ is a positive integer} \end{align} \)

\( \require{AMSsymbols} \begin{align} \text{For } n &= 3k, \\ n(n+1)(n+2) &= 3k(3k+1)(3k+2) \\ &= \color{red}3 \times k(3k+1)(3k+2) \\ &\leadsto \text{divisible by } 3 \\ \text{For } n &= 3k+1, \\ n(n+1)(n+2) &= (3k+1)(3k+2)(3k+3) \\ &= \color{red}3 \times (3k+1)(3k+2)(k+1) \\ &\leadsto \text{divisible by } 3 \\ \text{For } n &= 3k+2, \\ n(n+1)(n+2) &= (3k+2)(3k+3)(3k+4) \\ &= \color{red}3 \times (3k+2)(k+1)(3k+4) \\ &\leadsto \text{divisible by } 3 \\ \therefore n(n+1)(n+2) &\text{ is divisible by 3} \end{align} \)

(b) Hence, prove \( n^3 + 2n \) is divisible by \( 3 \) algebraically.

\( \require{AMSsymbols} \begin{align} n^3+2n &= n^3+3n^2+2n-3n^2 \\ &= n\left(n^2+3n+2\right)-3n^2 \\ &= n(n+1)(n+2)-3n^2 \\ &\leadsto \text{divisible by 3} &\color{green}{n(n+1)(n+2) \text{ is divisible by 3 and } 3n^2 \text{ is divisible by 3}} \end{align} \)

**Real-World Applications:**

The ability to prove divisibility by 3 using algebraic methods is not just a mathematical exercise. It has practical applications in various fields, including computer science, where it’s used in algorithms and data validation.

## Frequently Asked Questions

**Why is it useful to have a divisibility rule for \(3\) in algebra?**

*Answer*: A divisibility rule for 3 in algebra is incredibly useful because it lets you quickly determine whether a given algebraic expression or number is divisible by \(3\) without performing the actual division. This can save time in problem-solving and help you make quick mathematical decisions. It’s especially valuable when dealing with large algebraic expressions or complex equations.

**How does the divisibility rule for \(3\) work in algebra?**

*Answer*: The divisibility rule for 3 in algebra is based on the concept of the sum of digits. To determine if an algebraic expression is divisible by 3, add all the digits (coefficients and constants) in the expression. If the sum of these digits is divisible by 3, then the entire expression is divisible by 3. It’s a powerful shortcut that simplifies divisibility testing for 3 in algebraic contexts.

## Conclusion

In conclusion, the algebraic method for proving divisibility by \(3\) is valuable in mathematics and beyond. Understanding this approach enhances problem-solving skills and contributes to a deeper comprehension of number theory concepts.

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