Lesson 5

Representing Subtraction

Lesson Narrative

In this lesson, students represent a subtraction of signed numbers on a number line by relating it to an addition equation with a missing addend. The convention for representing subtraction on the number line fits with the convention for representing addition. When we represent \(a + b = c\), we represent \(a\) with an arrow starting at zero, \(b\) with an arrow starting where the first arrow ends, and \(c\) with a point at the end of the second arrow. So when we want to represent \(c -a = b\), we represent \(c\) with a point, \(a\) with an arrow starting at zero, and the difference \(b\) is the other arrow that is needed to reach from the end of the first arrow to the point. 

At the beginning of the lesson, students see that a subtraction equation like \(\text-8 - 3 = {?}\) can be thought of as the related addition equation \(3 + {?} = \text-8\). After repeatedly calculating differences this way (MP8), students recognize that the answer to each subtraction problem is the same number they would get by adding the opposite of the number. For example, by the end of the lesson, students see that \(\text-8 - 3 = {?}\) can also be thought of as \(\text-8 + \text-3 = {?}\)

Learning Goals

Teacher Facing

  • Generalize (orally and in writing) that subtracting a number results in the same value as adding the additive inverse.
  • Interpret a number line diagram that represents subtracting signed numbers as adding with an unknown addend.
  • Use a number line diagram to find the difference of signed numbers, and explain (orally) the reasoning.

Student Facing

Let's subtract signed numbers.

Learning Targets

Student Facing

  • I can explain the relationship between addition and subtraction of rational numbers.
  • I can use a number line to subtract positive and negative numbers.

CCSS Standards

Building On

Addressing

Building Towards

Glossary Entries

  • deposit

    When you put money into an account, it is called a deposit.

    For example, a person added \$60 to their bank account. Before the deposit, they had \$435. After the deposit, they had \$495, because \(435+60=495\).

  • withdrawal

    When you take money out of an account, it is called a withdrawal.

    For example, a person removed \$25 from their bank account. Before the withdrawal, they had \$350. After the withdrawal, they had \$325, because \(350-25=325\).