Addition and subtraction within 20 - Related facts - Presentation

The lesson on related facts in addition and subtraction within 20 is designed to deepen students' understanding of these fundamental operations by exploring the concept of the inverse. Students are encouraged to use their existing knowledge to calculate inverse operations and to understand what 'inverse' means, with examples to illustrate the concept. The lesson includes a variety of activities where students are asked to complete addition sentences and then write corresponding subtraction sentences, reinforcing the idea that addition and subtraction are related operations. For instance, if they know that 12 + 1 equals 13, they are prompted to find the missing number in a related addition sentence and then to write a subtraction sentence that reflects the same relationship between the numbers.

Further activities challenge students to explore why subtraction is not commutative, meaning the order of numbers cannot be swapped to achieve the same result, unlike addition. This is exemplified by the fact that while 15 - 3 equals 12, the reverse, 3 - 15, does not equal 12. Students complete sentences to demonstrate their understanding of this concept. Additionally, they engage with bar models to create both addition and subtraction sentences, which helps to visualize the relationship between numbers in a concrete way. The lesson also includes reasoning tasks where students create as many related addition and subtraction sentences as possible from given numbers and match number sentences to visual representations like ten frames. Through independent work and discussion, students are encouraged to identify patterns and articulate their understanding of related facts in both addition and subtraction, as well as the non-commutative nature of subtraction.