Grade 5
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When Students Ask

Negative Numbers

  • Why should I bother learning this?
    Students should realize that not all situations can be modeled by positive numbers. Many situations require numbers that indicate the opposite direction of another number, such as distances above and below sea level, moving forwards or backwards, and depositing and withdrawing money. Students are in the process of making the transition from arithmetic to algebra. A good understanding of negative numbers and integers is needed to be successful in algebra.
  • Why do we change the sign of the second number and add when we are subtracting integers?
    Use a number line to demonstrate the meanings of addition and subtraction. On a number line, negative numbers are to the left and positive numbers to the right. Addition means to move in the same direction as the number being added, while subtraction means to move in the opposite direction. Demonstrate on the number line that to find 5 – negative3, you need to move three units. Which direction? Since negative3 means "move left 3," but the subtraction sign means "go the opposite direction," you need to move 3 units to the right. Then show that this is the same answer you get by adding 5 + 3.
  • How do I know if the sum of two integers is positive or negative?
    Students should generalize that the sum of two positive integers will be positive and that the sum of two negative integers will be negative after having done several examples. It will probably be more difficult for them to make a conjecture about the sum when adding two numbers opposite in sign. By having them examine some specific cases, some students might be able to find that the integer which is farther from zero determines the sign of the sum. That is, if the negative integer is farther from zero than the positive integer, the sum is negative. For example, negative8 + 5 = negative3 because negative8 is farther from zero than 5 is. And if the positive integer is farther from zero than the negative integer, the sum is positive. For example, 6 + (negative2) = 4, since 6 is farther from zero thannegative2 is.
    Placing Negative Numbers
    on a Number Line
    Adding and Subtracting
    with Negative Numbers

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