2 ## SIGNED NUMBERS## Positive and negativeThe absolute value and the algebraic sign Subtracting a larger number from a smaller The algebraic definition of the negative of a number IN ARITHMETIC we cannot subtract a larger number from a smaller: 2 − 3. But in algebra we can. And to do it, we invent "negative" numbers. 2 − 3 = −1 ("Minus 1" or "Negative 1"). That is, since 2 − 2 = 0, then 2 − 3 is one "less" than 0. We call it −1. −1 is a signed number. Its algebraic sign is − ("minus") 1. What are the two parts of a signed number? To see the answer, pass your mouse over the colored area.
Its The algebraic sign of +3 ("plus 3" or "positive 3") is + , and its absolute value is 3. The algebraic sign of −3 ("negative 3" or "minus 3") is − . The absolute value of −3 is also 3. For better or for worse, the minus sign '−' is not only the sign of a negative number. It is also the sign for the operation of subtraction. Those are two completely different concepts. As for the algebraic sign + , normally we do not write it. The algebraic sign of 2, for example, is understood to be + . As for 0, it is useful to say that it has both signs: −0 = +0 = 0. When we place a number within vertical lines, |−3|, that signifies its absolute value.
Problem 1. Evaluate each of the following.
2. How do we subtract a larger number from a smaller? 5 − 8 1. What will be the sign of the answer? It would not be wrong to say that we cannot take 8 from 5. We can of course take 5 from 8 -- and that is what we 5 − 8 = −3. Even in algebra we can only do ordinary arithmetic. But then we must choose the correct sign. We may say that this is the first rule of signed numbers: To subtract a larger number from a smaller, 1 − 5 = −4. We actually do 5 − 1. It was in order to subtract a larger number from a smaller that negative numbers were invented. 3. What is the only difference between 8 − 5 and 5 − 8 ? The algebraic signs. They have the same absolute value. Problem 2. Subtract.
Problem 3. You have 20 dollars in the bank and you write a check for 25 dollars. Now what is your balance? 20.00 − 25.00 = −5.00 The number line What you see above is called the number line. The negative numbers fall to the left of 0. (We say that a negative number is We imagine every number to be on the number line. And so the fraction ½ will fall between 0 and 1; the fraction −½ is between 0 and −1; and so on. 4. What is an integer? Any positive or negative 0, 1, −1, 2, −2, 3, −3, etc. When we draw the number line, we typically place the integers. It is on the number line in fact that we begin to see the practical uses for signed numbers. In general, they show the "direction" of some quantity. That quantity might be temperture: more than or less than a certain temperature designated as 0. Or it might be the position or "address" of some object: left or right of some fixed position chosen as 0. Or it might be the time: before or after a certain moment that again is chosen as 0. Or, as we all know, negative numbers can indicate a balance in a checking account
Problem 4. A rocket is scheduled to launch at precisely 9:16 AM, which is designated a) What time is it at b) What time is it at c) What time is it at d) What is the value of e) What is the value of The negative of any number Every number will have a negative. The negative of 3, for example, will be found at the same distance from 0, but on the other side. It is −3. Now, what number is the negative of −3? The negative of −3 will be the same distance from 0 on the −(−3) = 3. "The negative of −3 is 3." This will be true for any number −(− "The negative of What is in the box is called a formal rule . This means that whenever we see something that looks like this -- −(− -- something that has that
For example, −(−12) = 12. To learn algebra is to learn its formal rules. For, what are calculations but writing things in a different form? In arithmetic, we rewrite 1 + 1 as 2. In algebra, we rewrite −(− See Lesson 6. Problem 5. Evaluate the following. a) −(−10) = 10 b) −(2 − 6) = 4 c) −(1 + 4 − 7) = 2 d) −(− The algebraic definition of the negative of a number Finally, the way we define a negative number in algebra is as follows. −5, for example, is that number which when added to 5 itself, results in 0. 5 + (−5) = 0. That is, to each number
Problem 6. What number do you have to add to 8 in order to get 0? −8 Problem 7. What number do you have to add to −6 in order to get 0? 6
Problem 8. What number is the negative of −
The negative of − That is, −(− Problem 9. If
then what is the relationship between
Problem 10. If you had to prove that
how would you do it?
Show that To prove a property of anything, whether in mathematics, logic or the law, we have simply to show that it satisfies the definition of that property. Next Lesson: Adding and subtracting signed numbers Copyright © 2017 Lawrence Spector Questions or comments? E-mail: themathpage@yandex.com Private tutoring available. |