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Negative numbers

Negative numbers

In winter, in Jerusalem, the thermometer can drop below zero: −2\displaystyle -2 degrees means two degrees below zero. The shore of the Dead Sea is about −430\displaystyle -430 metres high – 430 metres below sea level. And someone with −50\displaystyle -50 ₪ in the bank owes the bank 50 ₪. Numbers like these, smaller than zero, are called negative numbers.

The number line
Zero is neither positive nor negative.

The number line goes on to the left of zero, in steps of the same size: −1\displaystyle -1, −2\displaystyle -2, −3\displaystyle -3 …

To the right of zero are the positive numbers (they can also be written with a plus sign: +3\displaystyle +3 is 3\displaystyle 3), and to its left the negative numbers, always with a minus sign. Together they are called signed numbers: every number has a direction – right or left of zero – and a distance from zero.

Exercise 1

Which numbers are marked with letters?

A\displaystyle A

    / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

    / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

    B\displaystyle B

      / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

      / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

      C\displaystyle C

        / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

        / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

        D\displaystyle D

          / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

          / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

          Comparing numbers
          −5\displaystyle -5 is further from zero, but it is smaller than −2\displaystyle -2.

          On the number line, the further right a number is, the larger it is. So −2>−5\displaystyle -2>-5: the point −2\displaystyle -2 is to the right of the point −5\displaystyle -5. With temperatures it is clear: two degrees below zero is warmer than five degrees below zero.

          Every positive number is larger than zero, and zero is larger than every negative number: −100<0<1\displaystyle -100<0<1.

          Exercise 2

          Write >\displaystyle > or <\displaystyle < in each box.

          a. −3  □  2\displaystyle -3\;\square\;2 b. −7  □  −4\displaystyle -7\;\square\;-4 c. 0  □  −1\displaystyle 0\;\square\;-1 d. −1.5  □  −0.5\displaystyle -1.5\;\square\;-0.5 e. −10  □  −9\displaystyle -10\;\square\;-9 f. −12  □  −14\displaystyle -\frac{1}{2}\;\square\;-\frac{1}{4}

          Exercise 3

          Order from smallest to largest: 3\displaystyle 3, −8\displaystyle -8, 0\displaystyle 0, −2.5\displaystyle -2.5, 1\displaystyle 1, −3\displaystyle -3.

          Opposite numbers
          The opposite of the opposite is the number itself.

          3\displaystyle 3 and −3\displaystyle -3 are the same distance from zero – three steps – but on its two sides. Such numbers are called opposite numbers: −3\displaystyle -3 is the opposite of 3\displaystyle 3, and 3\displaystyle 3 is the opposite of −3\displaystyle -3.

          The opposite of a number is written with a minus in front of it: the opposite of 7\displaystyle 7 is −7\displaystyle -7, and the opposite of −4\displaystyle -4 is −(−4)\displaystyle -(-4), that is 4\displaystyle 4. So −(−4)=4\displaystyle {-(-4)}={4}. The opposite of 0\displaystyle 0 is 0\displaystyle 0 itself.

          Exercise 4

          What is the opposite of: a. 12\displaystyle 12 b. −6\displaystyle -6 c. −2.5\displaystyle -2.5 d. 34\displaystyle \frac{3}{4} e. −(−9)\displaystyle -(-9)?

          f. The distance between two opposite numbers on the number line is 10.
          What are the numbers?

          a

            / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

            / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

            b

              / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

              / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

              c

                / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

                / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

                d

                  / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

                  / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

                  e

                    / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

                    / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power

                    f
                    Exercise 5

                    In the morning the temperature on Mount Hermon was −4\displaystyle -4 degrees, and at noon 3\displaystyle 3 degrees.
                    When was it warmer?
                    By how many degrees did the temperature rise from morning to noon?
                    Count the steps on the number line.

                      / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power

                      / – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power