Writing Equations in Two Variables - Free Educational videos for Students in K-12 | Lumos Learning

## Writing Equations in Two Variables - Free Educational videos for Students in k-12

#### Writing Equations in Two Variables - By Anywhere Math

Transcript
00:0-1 Welcome to anywhere . Math . I'm Jeff , Jacobson
00:02 and Well let's see . We've written equations with one
00:06 variable . We've solved those equations using addition and subtraction
00:11 . We solve them using multiplication and division . Now
00:15 it's time to write equations with two variables . Let's
00:19 get started . So today we're writing equations in two
00:41 variables first . What exactly are we talking about when
00:44 we're saying equations in two variables ? Well uh equations
00:49 in two variables that represents two quantities that change in
00:53 relationship to one another . This word relationships really important
00:57 . What that means is as one of the variables
01:00 change . So does the other variable . They're related
01:04 to each other . You can't change one and not
01:06 the other um Or you can't change one and the
01:09 other one not be affected I should say . So
01:12 now . Well what exactly would a solution to an
01:15 equation like that look like ? Well you've got two
01:18 variables so you're gonna have to values in your solution
01:22 and that solution to an equation two variables is an
01:26 ordered parent . Okay . So commonly like an X
01:34 . Y . Okay . Where each one of these
01:38 uh values would be would work together as a solution
01:46 little bit more about what these values are called and
01:50 what they look like . So the two variables in
01:53 these types of equations have names . 1st 1 we
01:56 call the independent variable and that's just the quantity that
02:00 can change freely . Basically we choose what that value
02:04 is for that variable . Okay now the second variable
02:07 we call the dependent variable and just like the name
02:12 . The value depends on what the independent variable is
02:17 . Now that might be a little bit confusing but
02:19 for example here's my favorite mug . Uh If I
02:24 have coffee in this mug and I'm drinking some okay
02:28 and I ask you well how much coffee is left
02:32 you would say ? Well it depends it depends on
02:35 how much I drank already . Right ? So in
02:40 that situation uh the dependent variable would be , what
02:47 is it how much I drink or how much is
02:49 left ? Well the dependent variable would be how much
02:52 is left because how much is left depends on how
02:57 much I already drank , right ? The independent variable
03:01 would be how much I drank , right ? I
03:03 can decide that on my own . I can take
03:05 one sip , I could take two , I could
03:07 do the whole thing . Okay so how much I
03:10 drink would be independent variable . How much is left
03:13 would be the dependent variable because it depends on how
03:17 much I drink . So hopefully that helps keep them
03:19 clear . Now we can graph uh these types of
03:25 these variables and the solutions as an ordered pair ,
03:28 right ? We've graphed ordered pairs before if you're wondering
03:31 which one would be the X . Axis . Which
03:33 would be the Y . Typically uh the independent variable
03:38 , the one that you choose freely . That would
03:41 represent your ex value on the X axis . Independent
03:46 yes . Would be like your ex value and you're
03:49 dependent . Would then be your y depend . Okay
03:56 so if you think you have an order pair typically
03:58 you would say you're independent value first and then you're
04:01 dependent . Okay let's look at example . Alright example
04:05 one tell whether the order pair is a solution .
04:08 So for my first one why it was two X
04:11 . My order pair is 36 . So I'm gonna
04:13 see if that's a solution . If it's a solution
04:15 it should make this equation true . Just like a
04:18 solution to uh an equation with just one variable .
04:22 So to check all I have to do is substitute
04:26 this three is my ex value . So I'm going
04:30 to substitute that three in there and this six is
04:33 my why value ? So I'm gonna substitute in for
04:36 why ? So when I do that I'm gonna get
04:40 I'll do it in red six . And the question
04:45 is that equal to uh two times substitute ? I
04:52 didn't blue the three in for X . So two
04:54 times three . Right ? That's the question . Well
04:58 two times three is six so we have six is
05:01 equal to six . So yes 36 is a solution
05:07 to that . Uh that equation . Now is it
05:10 the only solution ? Right ? That's another question .
05:14 We won't get into that yet . Um But three
05:16 sixes a solution . So let's do the next one
05:19 . Why equals four X minus three . We're asking
05:22 is 4 12 that ordered pair . Is it a
05:24 solution ? You can pause the video and try it
05:27 on your own first . Um Here we go .
05:30 So four again I'm gonna substitute at that in for
05:33 my ex 12 . I'm gonna substitute in for my
05:35 why ? So I get 12 is equal to .
05:40 Well that's the question we're seeing . If it is
05:42 Four times four , make sure you use your parentheses
05:47 , right ? If you don't have them it's gonna
05:49 look like 44 uh minus three . Again . 12
05:54 . Is that equal to ? Well four times four
05:56 is 16 minus 3 . 16 minus three is 13
06:00 . So is that equal to 12 ? No they
06:03 are not equal . Which means 4 12 is not
06:08 a solution . Okay let's do another example . Alright
06:11 here we go . With example too . The equation
06:14 y equals 128 -8 x . gives the amount why
06:19 in fluid ounces of milk left in a gallon jug
06:23 after you pour X amount of cups . Okay uh
06:27 So first part A . Is identified the independent and
06:30 dependent variable . So again independent . That's the thing
06:34 that changes freely . That's the thing that we would
06:36 decide , dependent variables , depend on what you decide
06:42 with the independent what what you do first with the
06:44 independent variable ? Uh So sometimes when you're doing this
06:48 it's easier to find out the dependent variable first .
06:52 So in this situation uh First what are two variables
06:56 ? Well we've got Y . And X . Um
06:59 What does that X . Represent for X . Represents
07:02 the number of cups yeah . Of milk that we
07:09 pour out of the jug . Why represents how much
07:14 is left the amount why in food ounces of milk
07:16 left ? So I just write milk milk left .
07:23 So if I think about that Um you might notice
07:27 . Well then if we're subtracting eight X . Where
07:31 did that eight come from ? Right . For example
07:34 if we have one , we poured one cup .
07:37 We multiply by eight . How why would we multiply
07:40 by it ? If you look here why isn't fluid
07:43 ounces ? This isn't cups were multiplying the amount of
07:47 cups times eight . That's because there's eight fluid ounces
07:50 in a cup . Okay Um where is that ?
07:53 128 come from ? We're starting with that . We're
07:57 subtracting out how many cups . So if you think
07:59 well what if I didn't pour any any cups of
08:03 milk out ? Right ? So if that was zero
08:07 Um eight times zero is just zero . So we're
08:09 subtracting zero . That basically goes away and we have
08:12 128 . So that would be the milk left is
08:15 128 fluid ounces . That 128 represents how much is
08:20 in the jug in a full jug ? Right so
08:23 128 fluid ounces in one gallon jug . Okay um
08:28 So that's where those come from , that's where those
08:30 numbers come from , it . Sometimes it helps to
08:32 know that . So anyway let's get back to independent
08:35 dependent variables . What's the thing that depends on the
08:39 other thing ? Do the amount of cups that we
08:43 choose that we pour out ? Does that depend on
08:45 anything ? No that's we choose that up to ourselves
08:49 . How about how much milk is left ? Does
08:50 that depend on anything ? Well yeah , the amount
08:53 of milk left depends on how many cups of milk
08:56 we pour out . So that should tell you that
08:59 the dependent variable is the why ? Yeah . Uh
09:05 huh . Equals the Y . Which is the uh
09:09 the milk left ? Okay . And the independent ,
09:16 what we choose freely is how many cups we pour
09:19 out . So independent then . Yeah , that variables
09:25 the X . Variable . And that's the number of
09:29 cops that we pour out . Okay so that's part
09:32 A . Now part B . How much milk is
09:35 left after 10 cups ? Well again 10 cups that's
09:39 represented by X . In my variables . So If
09:43 X . is equal to 10 , all I need
09:44 to do to find out how much milk is left
09:47 is substitute . So I'm gonna have y . is
09:50 equal to 128 -8 . I'm gonna use my parentheses
09:56 . Uh eight times 10 cups . Well , order
10:01 of operations . I'm gonna do my multiplication first .
10:04 Why equals 1 . 28 minus eight times 10 is
10:07 80 . Now I've got my subtraction . Y .
10:10 Equals 48 . And units . Remember why was in
10:16 fluid ounces ? So 48 fluid ounces of milk left
10:23 . Okay . As pop . You can't I'm sorry
10:25 you can't see that . 48 fluid ounces left .
10:31 Okay so there's my final answer for being here is
10:35 something to try on your own . Thank you so
10:44 much for watching . And if you like this video
Summarizer

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