3.4: Addition of Signed Numbers
Overview
- Addition of Numbers with Like Signs
- Addition with Zero
- Addition of Numbers with Unlike Signs
Addition of Numbers with Like Signs
Let us add the two positive numbers \(2\) and \(3\). We perform this addition on the number line as follows.
We begin at \(0\), the origin.
Since \(2\) is positive, we move \(2\) units to the right.
Since \(3\) is positive, we move \(3\) more units to the right.
We are now located at \(5\).
Thus, \(2+3=5\).
Summarizing, we have:
(\(2\) positive units) + (\(3\) positive units) \(=\) (\(5\) positive units)
Now let us add the two negative numbers \(−2\) and \(−3\). We perform this addition on the number line as follows.
We begin at \(0\), the origin.
Since \(−2\) is negative, we move \(2\) units to the left.
Since \(−3\) is negative, we move \(3\) more units to the left.
We are now located at \(−5\).
Thus, \((−2)+(−3)=−5\).
Summarizing, we have
(\(2\) negative units)+(\(3\) negative units) \(=\) (\(5\)negative units)
These two examples suggest that:
(positive number) + (positive number) = (positive number)
(negative number) + (negative number) = (negative number)
To add two real numbers that have the same sign, add the absolute values of the numbers and associate the common sign with the sum.
Sample Set A
Find the sums.
\(3+7\)
Add these absolute values.
\(\left.\begin{array}{l}|3|=3 \\ |7|=7\end{array}\right\} \quad 3+7=10\)
The common sign is "+."
\(3+7=+10 \quad or \quad 3+7=10\)
\((-4) + (-9)\)
Add these absolute values.
\(\left.\begin{array}{l}|-4|=-4 \\ |-9|=-9\end{array}\right\} \quad 4+9=13\)
The common sign is "-."
\((-4) + (-9) = -13\)
Practice Set A
Find the sums.
\(8 + 6\)
- Answer
-
\(14\)
\(41 + 11\)
- Answer
-
\(52\)
\((-4) + (-8)\)
- Answer
-
\(-12\)
\((-36) + (-9)\)
- Answer
-
\(-45\)
\(-14 + (-20)\)
- Answer
-
\(-34\)
\(-\dfrac{2}{3} + (-\dfrac{5}{3})\)
- Answer
-
\(-\dfrac{7}{3}\)
\(-2.8 + (-4.6)\)
- Answer
-
\(-7.4\)
Addition with Zero
Notice that
Addition with 0
(0) + (a positive number) = (that same positive number)
(0) + (a negative number) = (that same negative number)
The Additive Identity Is 0
Since adding 0 to a real number leaves that number unchanged, 0 is called the additive identity .
Addition of Numbers with Unlike Signs
Now let us perform the addition \(2+(−6)\). These two numbers have unlike signs. This type of addition can also be illustrated using the number line.
We begin at \(0\), the origin.
Since \(2\) is positive, we move \(2\) units to the right.
Since \(−6\) is negative, we move, from the \(2\), \(6\) units to the left.
We are now located at \(−4\).
A rule for adding two numbers that have unlike signs is suggested by noting that if the signs are disregarded, \(4\) can be obtained from \(2\) and \(6\) by subtracting \(2\) from \(6\). But \(2\) and \(6\) are precisely the absolute values of \(2\) and \(−6\). Also, notice that the sign of the number with the larger absolute value is negative and that the sign of the resulting sum is negative.
To add two real numbers that have unlike signs, subtract the smaller absolute value from the larger absolute value and associate the sign of the number with the larger absolute value with this difference.
Sample Set B
Find the following sums.
\(7+(-2)\)
\(\underbrace{|7|=7}_{\text {Larger absolute value. }} \underbrace{|-2|=2}_{\text {Smaller absolute value. }}\)
Sign is "+".
Subtract absolute values: \(7-2=5\)
Attach the proper sign: "+"
\(7+(-2)=+5 \quad or \quad 7+(-2)=5\)
\(3+(-11)\)
\(\underbrace{|3|=3}_{\text {Smaller absolute value. }} \underbrace{|-11|=11}_{\text {Larger absolute value. }}\)
Sign is "-".
Subtract absolute values: \(11-3=8\)
Attach the proper sign: "-"
\(3+(-11)=-8\)
The morning temperature on a winter's day in Lake Tahoe was \(-12\) degrees. The afternoon temperature was \(25\) degrees warmer. What was the afternoon temperature?
We need to find \(-12 + 25\)
\(\underbrace{|-12|=12}_{\text {Smaller absolute value. }} \underbrace{|25|=25}_{\text {Larger absolute value. }}\)
Sign is "+".
Subtract absolute values: \(25 - 12 = 13\)
Attach the proper sign: "+"
\(-12 + 25 = 13\)
Thus, the afternoon temperature is \(13\) degrees.
Practice Set B
Find the sums.
\(4+(−3)\)
- Answer
-
\(1\)
\(−3+5\)
- Answer
-
\(2\)
\(15+(−18)\)
- Answer
-
\(-3\)
\(0+(−6)\)
- Answer
-
\(-6\)
\(−26+12\)
- Answer
-
\(-14\)
\(35+(−78)\)
- Answer
-
\(-43\)
\(15+(−10)\)
- Answer
-
\(5\)
\(1.5+(−2)\)
- Answer
-
\(-0.5\)
\(−8+0\)
- Answer
-
\(-8\)
\(0+(0.57)\)
- Answer
-
\(0.57\)
\(−879+454\)
- Answer
-
\(-425\)
\(−1345.6+(−6648.1)\)
- Answer
-
\(-7993.7\)
Exercises
Find the sums for the the following problems.
\(4+12\)
- Answer
-
\(16\)
\(8 + 6\)
\(6+2\)
- Answer
-
\(8\)
\(7 + 9\)
\((−3)+(−12)\)
- Answer
-
\(-15\)
\((−6)+(−20)\)
\((−4)+(−8)\)
- Answer
-
\(-12\)
\((−11)+(−8)\)
\((−16)+(−8)\)
- Answer
-
\(-24\)
\((−2)+(−15)\)
\(14+(−3)\)
- Answer
-
\(11\)
\(21+(−4)\)
\(14+(−6)\)
- Answer
-
\(8\)
\(18+(−2)\)
\(10+(−8)\)
- Answer
-
\(2\)
\(40+(−31)\)
\((−3)+(−12)\)
- Answer
-
\(-15\)
\((−6)+(−20)\)
\(10+(−2)\)
- Answer
-
\(8\)
\(8+(−15)\)
\(−2+(−6)\)
- Answer
-
\(-8\)
\(−11+(−14)\)
\(−9+(−6)\)
- Answer
-
\(-15\)
\(−1+(−1)\)
\(−16+(−9)\)
- Answer
-
\(-25\)
\(−22+(−1)\)
\(0+(−12)\)
- Answer
-
\(-12\)
\(0+(−4)\)
\(0+(24)\)
- Answer
-
\(24\)
\(-6 + 1 + (-7)\)
\(−5+(−12)+(−4)\)
- Answer
-
\(-21\)
\(−5+5\)
\(−7+7\)
- Answer
-
\(0\)
\(−14+14\)
\(4+(−4)\)
- Answer
-
\(0\)
\(9+(−9)\)
\(84+(−61)\)
- Answer
-
\(23\)
\(13+(−56)\)
\(452+(−124)\)
- Answer
-
\(328\)
\(636+(−989)\)
\(1811+(−935)\)
- Answer
-
\(876\)
\(−373+(−14)\)
\(−1221+(−44)\)
- Answer
-
\(−1265\)
\(−47.03+(−22.71)\)
\(−1.998+(−4.086)\)
- Answer
-
\(−6.084\)
\([(−3)+(−4)]+[(−6)+(−1)]\)
\([(−2)+(−8)]+[(−3)+(−7)]\)
- Answer
-
\(-20\)
\([(−3)+(−8)]+[(−6)+(−12)]\)
\([(−8)+(−6)]+[(−2)+(−1)]\)
- Answer
-
\(-17\)
\([4+(−12)]+[12+(−3)]\)
\([5+(−16)]+[4+(−11)]\)
- Answer
-
\(-18\)
\([2+(−4)]+[17+(−19)]\)
\([10+(−6)]+[12+(−2)]\)
- Answer
-
\(14\)
\(9+[(−4)+7]\)
\(14+[(−3)+5]\)
- Answer
-
\(16\)
\([2+(−7)]+(−11)\)
\([14+(−8)]+(−2)\)
- Answer
-
\(4\)
In order for a small business to break even on a project, it must have sales of $21,000. If the amount of sales was $15,000, how much money did this company fall short?
Suppose a person has $56.00 in his checking account. He deposits $100.00 into his checking account by using the automatic teller machine. He then writes a check for $84.50. If an error causes the deposit not to be listed into this person's account, what is this person's checking balance?
- Answer
-
\(-$28.50\)
A person borrows $7.00 on Monday and then $12.00 on Tuesday. How much has this person borrowed?
A person borrows $11.00 on Monday and then pays back $8.00 on Tuesday. How much does this person owe?
- Answer
-
\($3.00\)
Exercises for Review
Simplify \(\dfrac{4(7^2-6 \cdot w^3)}{2^2}\)
Simplify \(\dfrac{35a^6b^2c^5}{7b^2c^4}\)
- Answer
-
\(5a^6c\)
Simplify \((\dfrac{12a^8b^5}{4a^5b^2})^3\)
Determine the value of \(|-8|\)
- Answer
-
\(8\)
Determine the value of \((|2|+|4|^2)+|−5|^2\)