Subtraction of Integers

I can subtract positive and negative numbers.

Focus Question

What is the rule for subtracting positive and negative numbers?

The subtraction rule Rule: To subtract ADD the

OPPOSITE. Or – Subtracting is the same as

ADDING THE OPPOSITE.

5 + (-7) = 5 – 7 =

-2-2

Example: -5 - (-3) ADD the OPPOSITE –5 + (3) = -2

Notice the subtraction operation changed to addition, and

the opposite of –3 becomes 3.

NOTE: only the number behind the subtraction sign gets changed.

- 3 – 2

- ADD the OPPOSITE

- 3 – 2

- ADD the OPPOSITE –3 + (-2) = ____-5

Two Negatives 5 - (-3)

Two Negatives 5 - (-3) ADD the OPPOSITE 5 + (+3) Notice the use of the rule. 5 - (-3) = 5 + 3 = 8

-8 –(-4) + 2

ADD the OPPOSITE

-8 –(-4) + 2

ADD the OPPOSITE –8 + (+4) + 2. -8 - (-4) +2 - 8 + 4 + 2 = - 2

Try some with the rule

-8 – (-10)= 4 - 7 = -9 - 5 = -20 – (-4) = 40 – 32 =

Try some with the rule

-8 – (-10)= 4 - 7 = -9 - 5 = -20 – (-4) = 40 – 32 =

-8 + 10 =4 + (-

7)=-9 + (-5)=-20 +

4=40 + (-32)=

2

-3-14

-168

Example: -7 – 9 - (-4) – 3 + 9 - (-4)=

Example: -7 – 9 - (-4) – 3 + 9 - (-4)= Changes on paper to –7 + -9 + 4 + - 3 + 9 + 4 =

Then think DOWN 7, Down 9 more, then UP 4, Down 3, UP 9, and UP 4.

The result is –2.

EXAMPLE 2 Evaluating a Variable Expression

Evaluate 15 – a – b when a = 24 and b = – 36 .

EXAMPLE 2 Evaluating a Variable Expression

Evaluate 15 – a – b when a = 24 and b = – 36 .

15 – 24 – (–36) =15 + (–24) + 36 =

–9 + 36 =

27

Add the opposite of 24 and of –36.

Add –9 and 36.

Substitute 24 for a and –36 for b.

Add 15 and –24.

15 – a – b =

GUIDED PRACTICE for Examples 1, 2, and 3

a – b – (–6) a = 8, b = –15

GUIDED PRACTICE for Examples 1, 2, and 3

8 – (–15) – (-6)

8 + 15 + 6

29

Add the opposite of –15 andof –6.

Substitute 8 for a and –15 for b.

Add.

a – b – (–6) a = 8, b = –15

In the Sahara Desert one day it was 136°F.

In the Gobi Desert a temperature of -50 F was recorded.

What is the difference between these two temperatures?

Write a subtraction expression to represent this situation.

Assignment: No Calculators - show your work