Multiplication of Integers

Chapter 2.4

Algebra I

Pg 73-77

Warm-Up

3.8 4.1 3 1

5 6

6.8 0.4

1 1

10 4

Warm-Up answers

3.8 4.1 15.58 3 1 3 1

5 6 30 10

6.8 0.4 17

1 1 1 4

10 4 10 1

4 2

10 5

What’s the Deal?

In today’s lesson we will review multiplication and division of signed numbers.

What and why?

7 x -5 = - 35

+7 x +5 = +35

-7 x 5 = - 35

-7 x -5 = + 35

Multiplication is fast adding.

-5 + -5 + -5 + -5 + -5 + -5 + -5 = -35

5 + 5 + 5 + 5 + 5 + 5 + 5 = 35

-7 + -7 + -7 + -7 + -7 = - 35

- (-7 + -7 + -7 + -7 + -7) = - (-35)

Count the Negatives

7 x -5 = - 35+7 x +5 = +35-7 x 5 = - 35

-7 x -5 = + 35

Count the negative signs..Ask Odd or Even?

• Odd?Negative

Answer

• Even?Positive

Answer

This works the same for division.

Number Families

Factor x Factor = Product

F x F = P

Product / Factor = Factor

P / F = F

Number Families

-8 * 5 = -40

-5 * 8 = -40

-40 / 5 = -8

-40 / 8 = -5

-8 * -5 = 40

-5 * -8 = 40

40 / -5 = -8

40 / -8 = -5

Try Some

-11 * +5 =

-7 * -10 =

+6 * -5 =

-7 * +8 =

-35 / -5 =+63 / -9 =-56 / -7 =+32 / -8 =

Try Some - answers

-11 * +5 = - 55

-7 * -10 = + 70

+6 * -5 = - 30-7 * +8 = - 56

-35 / -5 = +7+63 / -9 = -7-56 / -7 = +8+32 / -8 = -4

Part Two

Fractions and Reciprocals

Multiply fractions

3 1

5 6

2 1

7 3

7 12

8 23

2

21

1

2

1

10

3

2

21

46

Reciprocals are used for dividing fractions.

What’s a reciprocal?

Multiply a number by its reciprocal to get an answer of +1

2

33

2 6

16

What’s the reciprocal of

7/8?

8/7

Using Reciprocals

• Reciprocals are useful for dividing fractions.

• Rewrite the division of a fraction using multiplication by the reciprocal of the 2nd number.

1 1

3 2

1 1 1 2

3 2 3 1

Rewrite these number sentences.

4 2

5 7

296

3

Rewritten number sentences

4 2 4 7 28 14

5 7 5 2 10 5

2 96 396 144

3 1 2

One is your friend.Use 1 to make whole numbers

into fractions.