1
00:00:00,568 --> 00:00:02,323
Now, let's think about expressions

2
00:00:02,323 --> 00:00:04,079
with more than one variable.

3
00:00:04,125 --> 00:00:06,009
So, let's say I had the expression

4
00:00:06,040 --> 00:00:09,325
"a + b"

5
00:00:09,325 --> 00:00:11,222
and I were to ask you

6
00:00:11,222 --> 00:00:13,042
what is this expression equal to

7
00:00:13,042 --> 00:00:16,000
when "a" is equal to 5

8
00:00:16,000 --> 00:00:19,574
and "b" is equal to 7

9
00:00:19,574 --> 00:00:23,789
Well, in this situation, this expression would be...

10
00:00:23,789 --> 00:00:25,635
instead of an "a" we would have a 5

11
00:00:25,635 --> 00:00:28,004
it would be 5 plus...

12
00:00:28,004 --> 00:00:31,821
instead of a "b", we would have a 7.

13
00:00:31,821 --> 00:00:36,169
So it would be 5 + 7 is equal to 12.

14
00:00:36,214 --> 00:00:38,111
Now let's try another expression.

15
00:00:38,111 --> 00:00:39,751
Let's say we had the expression

16
00:00:39,751 --> 00:00:54,102
"10x - 3y"

17
00:00:54,102 --> 00:00:55,401
And, let's think about

18
00:00:55,401 --> 00:00:57,594
what would this expression evaluate to

19
00:00:57,641 --> 00:01:01,187
when "x" is equal to - 1

20
00:01:01,187 --> 00:01:06,244
and when "y" is equal to 5.

21
00:01:06,291 --> 00:01:07,849
Well, let's just evaluate it.

22
00:01:07,849 --> 00:01:09,998
We have 10 times...

23
00:01:09,998 --> 00:01:10,825
instead of an "x",

24
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we know now that "x" is equal to -1,

25
00:01:13,471 --> 00:01:21,877
it would be 10 (-1) - 3 (5).

26
00:01:21,877 --> 00:01:23,338
So three times...

27
00:01:23,338 --> 00:01:25,630
instead of a "y", we will put a 5 there.

28
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- 3 (5).

29
00:01:28,268 --> 00:01:32,503
And so 10 (-1), that is -10

30
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and we're going to subtract from that 3 (5).

31
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You want to do the multiplication first

32
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by order of operations

33
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that's the convention.

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3 times 5 is 15.

35
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So, it is -10 - 15,

36
00:01:46,656 --> 00:01:52,256
which is equal to -25.

37
00:01:52,256 --> 00:01:54,807
And now, you have the next exercise

38
00:01:54,807 --> 00:01:57,992
to practice some of these a little bit more.