Divisibility Rule: Does Being Divisible by 9 Imply Divisibility by 3?

Divisibility Rules with Factor Relationships

Will a number divisible by 9 necessarily be divisible by 3?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Is every number divisible by 9 also divisible by 3?
00:04 A number whose sum of digits is divisible by 9 is divisible by 9
00:25 Let's break down 9 into factors 3 and 3
00:38 Therefore, the number must also be divisible by 3
00:42 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Will a number divisible by 9 necessarily be divisible by 3?

2

Step-by-step solution

To solve this problem, we need to apply the divisibility rules for both 9 and 3.

  • Step 1: Understand divisibility by 9.
    A number is divisible by 9 when the sum of its digits is a multiple of 9.
  • Step 2: Understand divisibility by 3.
    A number is divisible by 3 when the sum of its digits is a multiple of 3.
  • Step 3: Relate the rules.
    Since any multiple of 9 (given by the sum of digits) is also a multiple of 3, a number divisible by 9 is necessarily divisible by 3.

Therefore, it follows that if a number is divisible by 9, it must be divisible by 3, because the divisibility by 9 inherently satisfies the divisibility condition for 3.

Thus, the correct answer is Yes.

3

Final Answer

Yes

Key Points to Remember

Essential concepts to master this topic
  • Rule: If 9 divides a number, then 3 automatically divides it
  • Technique: Since 9 = 3 × 3, every multiple of 9 contains 3 as factor
  • Check: Test with 18: divisible by 9 (1+8=9) and by 3 (1+8=9, 9÷3=3) ✓

Common Mistakes

Avoid these frequent errors
  • Thinking divisibility rules work independently
    Don't assume divisibility by 9 and 3 are separate conditions = missing the factor relationship! This ignores that 9 = 3 × 3, so 9's factors include 3. Always remember that divisibility by a larger number implies divisibility by its factors.

Practice Quiz

Test your knowledge with interactive questions

Determine if the following number is divisible by 3:

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FAQ

Everything you need to know about this question

Why does being divisible by 9 guarantee divisibility by 3?

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Because 9 = 3 × 3! When a number is divisible by 9, it means 9 goes into it evenly. Since 9 contains the factor 3, that factor of 3 must also divide the number.

Does this work the other way around - if divisible by 3, then by 9?

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No! A number divisible by 3 is not necessarily divisible by 9. For example, 12 is divisible by 3 (1+2=3) but not by 9 (1+2=3, which is not a multiple of 9).

How can I remember this rule?

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Think of it like family relationships: if you're a grandchild, you're automatically a child too. Similarly, if a number is in the "9 family," it's automatically in the "3 family" because 3 is a factor of 9.

What other numbers follow this same pattern?

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This works for any factor relationship! Numbers divisible by 6 are also divisible by 3 and 2. Numbers divisible by 12 are also divisible by 6, 4, 3, and 2. Always check if one number is a factor of another!

Can I use the digit sum rule to check both 3 and 9?

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Yes! Add up all digits. If the sum is a multiple of 9, the number is divisible by both 9 and 3. If the sum is only a multiple of 3 (like 6, 12, 15), it's divisible by 3 but not 9.

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