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K.2 Use addition properties to complete equations

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What are addition properties?

Addition properties are special rules about numbers that are always true when we add. These rules help us understand how numbers work together and make solving problems easier.

Example:
  • If we know that 3 + 5 = 8, then the properties tell us that 5 + 3 is also 8, even without counting again.
Note

Properties are like superpowers for numbers—they work every single time!

The commutative property of addition

The Commutative Property means we can change the order of the numbers we are adding, and the sum will stay the same.

Examples:
  • 4 + 7 = 11 and 7 + 4 = 11
  • 23 + 15 = 38 and 15 + 23 = 38
  • 156 + 209 = 365 and 209 + 156 = 365
Note

Think of "commutative" like a commute to school. You can take different roads, but you still get to the same place.

The associative property of addition

The Associative Property means when we are adding three or more numbers, we can group them in different ways, and the sum will stay the same.

Examples:
  • (2 + 3) + 4 = 5 + 4 = 9
  • 2 + (3 + 4) = 2 + 7 = 9
  • (10 + 5) + 20 = 15 + 20 = 35
  • 10 + (5 + 20) = 10 + 25 = 35
Note

The parentheses ( ) show which numbers to add first. "Associative" reminds us how we associate, or group, our friends together.

The identity property of addition

The Identity Property means if we add zero to any number, the number stays the same. Zero is called the "identity" because it doesn't change the number's identity.

Examples:
  • 8 + 0 = 8
  • 0 + 42 = 42
  • 567 + 0 = 567
  • 0 + 1,894 = 1,894
Note

Zero is like a mirror in addition—it shows you the number exactly as it is.

Using properties to complete equations

We can use our knowledge of addition properties to find missing numbers in equations. The property tells us what must be true.

Examples:
  • Commutative: If 15 + ▢ = 28, then ▢ + 15 = 28. The missing number is the same in both spots.
  • Associative: (7 + ▢) + 6 = 7 + (4 + 6). The missing number must be 4 to make both sides equal.
  • Identity: 329 + ▢ = 329. The missing number must be 0.
Note

Before you solve, ask yourself: "Which property is this equation showing?" This will help you find the pattern.