Add and Subtract Negatives

Click on the image at right to get the lesson app with instructor notes. Or you can install right from here by clicking the logo below: In this lesson, students learn about the Slingshot Method for adding and subtracting with negative integers. Students get a great deal of practice in simply adding and subtracting negative integers and then extend that knowledge briefly at the end to apply adding and subtracting to negative decimals as well. Students practice integer addition and subtraction with the help of an interactive number line, which uses the Slingshot Method to calculate sums and differences. Students will also see the concept that adding a negative is the same as subtracting a positive and subtracting a negative is the same as adding a positive. Students will solve different types of word problems that involve adding or subtracting (or both) negative integers. The scenarios used are temperature, stock prices, negative account balances, and being above or below a fundraising goal.

Module 1 Video

This video shows a method called the Slingshot Method. This method encourages students to read an addition sentence as they normally would—as moving to the right on the number line. When the addition involves a negative, the right movement turns into a “pulling back” like on a slingshot, which, when released, moves in the opposite direction the number of units described by the (absolute value of the) second number.

Module 2 Interactive

To use the interactive tool, use your mouse to click on the rectangle marker and then, while holding down the mouse, drag to the left (subtraction) or right (addition). This draws out a kind of pull string which will generate a sum or difference once you release the mouse. The addition or subtraction expression above the number line changes as you move the mouse, to show you what sum or difference you will create. Releasing the mouse will move the rectangle marker to the sum or difference specified above the number line. This expression remains until you click and drag the mouse again.