CCSS.MATH.CONTENT.1.OA.B.3
Apply properties of operations as strategies to add and subtract.2 Examples: If 8 + 3 = 11 is known, then 3 + 8 = 11 is also known. (Commutative property of addition.) To add 2 + 6 + 4, the second two numbers can be added to make a ten, so 2 + 6 + 4 = 2 + 10 = 12. (Associative property of addition.)
Apply properties of operations as strategies to add and subtract.2 Examples: If 8 + 3 = 11 is known, then 3 + 8 = 11 is also known. (Commutative property of addition.) To add 2 + 6 + 4, the second two numbers can be added to make a ten, so 2 + 6 + 4 = 2 + 10 = 12. (Associative property of addition.)
Students need to understand the commutative property of addition to make the two equations. For subtraction, they should know how to use the three numbers to make two different equations. Both of these are also related, and they need to notice this and understand why. When adding with three numbers students need to draw on their basic understanding of addition and know that these are the same skills.
CCSS.MATH.CONTENT.1.OA.B.4
Understand subtraction as an unknown-addend problem. For example, subtract 10 – 8 by finding the number that makes 10 when added to 8.
Understand subtraction as an unknown-addend problem. For example, subtract 10 – 8 by finding the number that makes 10 when added to 8.
When learning about fact families students will need to use their unknown-addend knowledge to figure out the remaining numbers, after placing the largest number. Students will also use their knowledge of adding within 20 when adding three numbers.