 ## ← Multiplying and dividing negative numbers

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Multiplying and dividing negative numbers

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Showing Revision 12 created 06/11/2012 by michal.stevove.

1. Welcome to the presentation
on multiplying and
2. dividing negative numbers.
3. Let's get started.
4. I think you're going to find
that multiplying and dividing
5. negative numbers are a lot
easier than it might
6. look initially. You just have to
remember a couple of rules.
7. And I am going to teach probably
in the future like I'm actually going
8. to give you more intuition on
why there rules work.
9. So the basic rules are when you
multiply two negative numbers,
10. so let's say I had negative
2 times negative 2.
11. First you just look at each
of the numbers as if there
12. was no negative sign.
13. Well you say well, 2
times 2 that equals 4.
14. And it turns out that if you
have a negative times a
15. negative, that that
equals a positive.
16. So let's write that
first rule down.
17. A negative times a negative
equals a positive.

18. What if it was negative
2 times positive 2?
19. Well in this case, let's
first of all look at the
20. two numbers without signs.
21. We know that 2 times 2 is 4.
22. But here we have a negative
times a positive 2, and it
23. turns out that when you
multiply a negative times a
24. positive you get a negative.
25. So that's another rule.
26. Negative times positive
is equal to negative.
27. What happens if you have a
positive 2 times a negative 2?
28. I think you'll probably guess
this one right, as you can tell
29. that these two are pretty much
the same thing by, I believe
30. it's the transitive property --
no, no I think it's the
31. communicative property.
32. I have to remember that.
33. But 2 times negative 2, this
also equals negative 4.
34. So we have the final rule that
a positive times a negative
35. also equals the negative.

36. And actually these second
two rules, they're kind
37. of the same thing.
38. A negative times a positive
is a negative, or a positive
39. times a negative is negative.
40. You could also say that as when
the signs are different and
41. you multiply the two numbers,
you get a negative number.
42. And of course, you already know
what happens when you have a
43. positive times a positive.
44. Well that's just a positive.
45. So let's review.
46. Negative times a
negative is a positive.
47. A negative times a
positive is a negative.
48. A positive times a
negative is a negative.
49. And positive times each
other equals positive.
50. I think that last little bit
completely confused you.
51. Maybe I can simplify
it for you.
52. What if I just told you if when
you're multiplying and they're
53. the same signs that gets
you a positive result.

54. And different signs gets
you a negative result.
55. So that would be either, let's
say a 1 times 1 is equal to 1,
56. or if I said negative 1 times
negative 1 is equal to
57. positive 1 as well.
58. Or if I said 1 times negative
1 is equal to negative 1, or
59. negative 1 times 1 is
equal to negative 1.
60. You see how on the bottom two
61. signs, positive 1
and negative 1?
62. And the top two problems,
this one right here
63. both 1s are positive.
64. And this one right here
both 1s are negative.
65. So let's do a bunch of problems
now, and hopefully it'll hit
66. the point home, and you also
could try to do along the
67. practice problems and also give
the hints and give you what rules to you so that should help you as well
68. So if I said negative 4 times
positive 3, well 4 times
69. 3 is 12, and we have a
negative and a positive.
70. So different signs
mean negative.
71. So negative 4 times
3 is a negative 12.
72. That makes sense because we're
essentially saying what's
73. negative 4 times itself three
times, so it's like negative 4
74. plus negative 4 plus negative
4, which is negative 12.
75. If you've seen the video on
76. numbers, you probably
should watch first.
77. Let's do another one.
78. What if I said minus
2 times minus 7.
79. And you might want to pause the
video at any time to see if you
80. know how to do it and
then restart it to see
82. Well, 2 times 7 is 14, and we
have the same sign here, so
83. it's a positive 14 -- normally
you wouldn't have to write the
84. positive but that makes it a
little bit more explicit.
85. And what if I had -- let me
think -- 9 times negative 5.
86. Well, 9 times 5 is 45.
87. And once again, the signs are
different so it's a negative.
88. And then finally what if it I
had -- let me think of some
89. good numbers -- minus
6 times minus 11.
90. Well, 6 times 11 is 66 and
then it's a negative and
91. negative, it's a positive.
92. Let me give you a
trick problem.
93. What is 0 times negative 12?
94. Well, you might say that the
signs are different, but
95. 0 is actually neither
positive nor negative.
96. And 0 times anything
is still 0.
97. It doesn't matter if the thing
you multiply it by is a
98. negative number or
a positive number.
99. 0 times anything is still 0.
100. So let's see if we can apply
these same rules to division.
101. It actually turns out that
the same rules apply.
102. If I have 9 divided
by negative 3.
103. Well, first we say
what's 9 divided by 3?
104. Well that's 3.
105. And they have different signs,
positive 9, negative 3.
106. So different signs
means a negative.
107. 9 divided by negative 3
is equal to negative 3.
108. What is minus 16 divided by 8?
109. Well, once again, 16
divided by 8 is 2, but
110. the signs are different.
111. Negative 16 divided by positive
8, that equals negative 2.
112. Remember, different signs will
get you a negative result.
113. What is minus 54
divided by minus 6?
114. Well, 54 divided by 6 is 9.
115. And since both terms, the
divisor and the dividend, are
116. both negative -- negative 54
and negative 6 -- it turns out
117. that the answer is positive.
Remember, same signs
118. result in a positive sign.
119. Let's do one more.
120. Obviously, 0 divided by
anything is still 0.
121. That's pretty straightforward.
122. And of course, you can't
divide anything by 0
123. -- that's undefined.
124. Let's do one more.
125. What is -- I'm just going to
think of random numbers --
126. 4 divided by negative 1?
127. Well, 4 divided by 1 is 4,
but the signs are different.
128. So it's negative 4.
129. I hope that helps.
130. Now what I want you to do is
actually try
131. as many of these multiplying and dividing
negative numbers as you can.
132. And you click on hints
133. and it'll remind you of which rule to use.
134. In your own time you might want