Lesson+6

media type="file" key="SubtractionMissionLesson6.mp3" width="240" height="20" (Click here to listen to the text of the lesson.)
 * Lesson 6: Mission completed? Don’t be deceived! **

And finally you think you are there, you have completed the subtraction mission. But can you trust the result? Some results can be deceiving and you need to make sure you have not been tricked.

This lesson will provide you with a last secret weapon. Once you have it, no one can beat you during any subtraction mission. To make sure that your result is reasonable, you need to be able to identify positive numbers and be able to apply a strategy to verify your result. Read and listen carefully to find out how to do it!



** Recall: ** You already know that there are two types of numbers: positive numbers and negative numbers. The following graph will help you to remember the difference:



** Process 1: ** The results of a subtraction problem (for your kind of mission in grade 3) can only be a positive number. Look at the following examples and non-examples:



** Practice: **

Practice identifying reasonable results by looking at the following examples. Just have a quick peek at the resulting number and then give your answer. Write it into your Mission Notebook.

Compare your answers with those on the **Lesson Exercise Keys** page, **Exercise 10**. Do you feel confident?

** Process 2: ** Besides subtraction, there are other number operations you have already learned, such as addition for example. Do you know that these skills can help you in verifying the result of a subtraction problem?

At the completion of each subtraction, you should verify your result before considering the mission completed. The following video will show you how it is done:

media type="youtube" key="n9E720Xvbic" height="349" width="560"

The procedure’s steps are therefore as follows: (1) Add the subtrahend to the answer. (2) Check whether the result from your addition leads to the minuend of the subtraction problem.

Can you think of a strategy that will help you to remember how to verify a result? How do you like this one? ** “Answer and subtrahend lead to minuend.” **

** Practice 2: ** Apply the procedure you have just learned to verify the results of the following subtraction algorithms:

Write everything into your mission notebook. Then compare your answers with those on the **Lesson Exercise Keys** page, **Exercise 11**.

How did you do? If you are confident, move on. If you feel you need more practice, re-do the examples.

** Conclusion: ** Agent X, you have completed the penultimate task and acquired another important secret weapon for your mission. Remember how to verify whether the mission result can be trusted:

(1) Determine whether the resulting number is positive. (2) Verify the result by adding the subtrahend to the answer.



** Assessment: ** Go ahead and earn another star on your special agent badge by completing the following assessment.

(1) Examine the following completed subtraction problems. Write the examples with correct answers into your Mission Notebook.



(2) Which of the following numbers are possible results of a subtraction problem? Include an explanation for your answers in your Mission Notebook.

(a) 56 (b) 84 (c) -12 (d) -39 (e) 27

(3) Do you know how to verify the result of a subtraction problem? If yes, please explain by using the following example: 76 - 12 = 64 Write it into your Mission Notebook.

Go to the **Lesson Exercise Keys** page. Open **Exercise 12** and compare your answers. If you have all answers correct, add another star to your badge in the Mission Notebook. If you need more practice, please repeat the lesson tomorrow before you work on Lesson 7.

Click here to go to Lesson 7.