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On Monday, Marty borrows $50.00 from his father. On Thursday, he gives his father$28.00. Can you write an addition statement to describe Marty’s financial transactions?

Guidance

When adding integers, you need to make sure you follow two rules:

1. Integers with unlike signs must be subtracted. The answer will have the same sign as that of the higher number.
2. Integers with the same sign must be added. The answer will have the same sign as that of the numbers being added.

In order to understand why these rules work, you can represent the addition of integers with manipulatives such as color counters or algebra tiles. A number line can also be used to show the addition of integers. The following examples show how to use these manipulatives to understand the rules for adding integers.

Example A

$5+(-3)=?$

Solution: This problem can be represented with color counters. In this case, the red counters represent positive numbers and the yellow ones represent the negative numbers.

One positive counter and one negative counter equals zero because $1+(-1)=0$ . Draw a line through the counters that equal zero.

The remaining counters represent the answer. Therefore, $5+(-3)=2$ . The answer is the difference between 5 and 3. The answer takes on the sign of the larger number. In this case, the five has a positive value and it is greater than 3.

Example B

$4+(-7)=?$

Solution:

Draw a line through the counters that equal zero.

The remaining counters represent the answer. Therefore, $4+(-7)=-3$ . The answer is the difference between 7 and 4. The answer takes on the sign of the larger number, which is 7 in this case.

Example C

$6x+(-8x)=?$

Solution: This same method can be extended to adding variables. Algebra tiles can be used to represent positive and negative values.

The green algebra tiles represent positive $x$ and the white tiles represent negative $x$ . There are 6 positive $x$ tiles and 8 negative $x$ tiles .

The remaining algebra tiles represent the answer. There are two negative $x$ tiles remaining. Therefore, $(6x)+(-8x)=-2x$ . The answer is the difference between $8x$ and $6x$ . The answer takes on the sign of the larger coefficient, which in this case is 8.

Example D

$(-3)+(-5)=?$

Solution: You can solve this problem with a number line. Indicate the starting point of -3 by using a dot. From this point, add a -5 by moving five places to the left. You will stop at -8.

The point where you stopped is the answer to the problem. Therefore, $(-3)+(-5)=-8$

Concept Problem Revisited

On Monday, Marty borrows $50.00 from his father. On Thursday, he gives his father$28.00.

Marty borrowed $50.00 which he must repay to his father. Therefore Marty has $-\50.00$ . He returns$28.00 to his father. Now Marty has $-\50.00+(\28.00)=-\22.00$ . He still owes his father \$22.00.

Guided Practice

1. $(-7)+(+5)=?$

2. $8+(-2)=?$

3. Determine the answer to $(-6)+(-3)=?$ and $(2)+(-5)=?$ by using the rules for adding integers.

1. $(-7)+(+5)=5-7=-2$

2. $8+(-2)=8-2=6$

3. $(-6)+(-3)=-9$ .

$(2)+(-5)=2-5=-3$ .

Explore More

Complete the following addition problems using any method.

1. $(-7)+(-2)$
2. $(6)+(-8)$
3. $(5)+(4)$
4. $(-7)+(9)$
5. $(-1)+(5)$
6. $(8)+(-12)$
7. $(-2)+(-5)$
8. $(3)+(4)$
9. $(-6)+(10)$
10. $(-1)+(-7)$
11. $(-13)+(9)$
12. $(-3)+(-8)+(12)$
13. $(14)+(-6)+(5)$
14. $(15)+(-8)+(-9)$
15. $(7)+(6)+(-9)+(-8)$

For each of the following models, write an addition problem and answer the problem.

1. .

1. .

1. .

1. .

1. .

Jan 16, 2013

Feb 26, 2015