0:00:01.050,0:00:04.040 Welcome to the presentation on[br]systems of linear equations. 0:00:04.040,0:00:06.970 So let's get started and[br]see what it's all about. 0:00:06.970,0:00:10.110 So let's say I had[br]two equations now. 0:00:10.110,0:00:15.740 The first equation let[br]me write it as 9x minus 0:00:15.740,0:00:21.760 4y equals minus 78. 0:00:21.760,0:00:28.950 And the second equation[br]I will write as 4x plus 0:00:28.950,0:00:33.390 y is equal to mine 18. 0:00:33.390,0:00:35.411 Now what we're going to do now[br]is we're actually going to 0:00:35.411,0:00:39.700 use both equations to[br]solve for x and y. 0:00:39.700,0:00:41.900 We already know that if you[br]have one equation, it has one 0:00:41.900,0:00:44.280 variable, it is very easy to[br]solve for that one variable. 0:00:44.280,0:00:45.790 But now we have to equations. 0:00:45.790,0:00:47.340 You can almost view them[br]as two constraints. 0:00:47.340,0:00:50.340 And we're going to solve[br]for both variables. 0:00:50.340,0:00:51.790 And you might be a[br]little confused. 0:00:51.790,0:00:52.520 How does that work? 0:00:52.520,0:00:54.910 Is it just magic that two[br]equations can solves 0:00:54.910,0:00:55.900 for two variables? 0:00:55.900,0:00:56.800 Well it's not. 0:00:56.800,0:00:58.850 Because you can actually[br]rearranged each of these 0:00:58.850,0:01:01.840 equations so that they[br]look kind of in normal y 0:01:01.840,0:01:03.700 equals mx plus b format. 0:01:03.700,0:01:06.200 And I'm not going to draw these[br]actual two equations because I 0:01:06.200,0:01:08.860 don't know what they look like,[br]but if this was a coordinate 0:01:08.860,0:01:11.620 axis-- and I don't know what[br]that first line actually does 0:01:11.620,0:01:14.010 look like, we could do another[br]model where we figured it out 0:01:14.010,0:01:16.500 --but lets just say for sake of[br]argument, that first line all 0:01:16.500,0:01:20.540 the x's and y's that satisfy 9x[br]minus 4y equals negative 0:01:20.540,0:01:22.690 78, let's say it looks[br]something like that. 0:01:22.690,0:01:26.400 And let's say all of the x's[br]and y's that satisfy that 0:01:26.400,0:01:31.340 second equation, 4x plus y[br]equals negative 18, let's say 0:01:31.340,0:01:34.680 that looks something like this. 0:01:34.680,0:01:35.620 Right? 0:01:35.620,0:01:40.050 So, on the line is all of the[br]x's and y's that satisfy this 0:01:40.050,0:01:42.555 equation, and on the green[br]line are all the x's and y's 0:01:42.555,0:01:44.275 that satisfy this equation. 0:01:44.275,0:01:48.170 But there's only one pair of[br]x and y's that satisfy both 0:01:48.170,0:01:51.430 equations, and you can guess[br]where that is, that's 0:01:51.430,0:01:52.560 right here right. 0:01:52.560,0:01:57.660 Whatever that point is-- I'll[br]do it in pink for emphasis. 0:01:57.660,0:02:00.800 Whatever this point is,[br]notice it's on both lines. 0:02:00.800,0:02:05.260 So whatever x and y that is[br]would be the solution to 0:02:05.260,0:02:06.670 this system of equations. 0:02:06.670,0:02:09.860 So let's actually figure[br]out how to do that. 0:02:09.860,0:02:12.080 So what we want to do is[br]eliminate a variable, because 0:02:12.080,0:02:15.200 if you can eliminate a variable[br]then we can just solve for 0:02:15.200,0:02:16.430 the one that's left over. 0:02:16.430,0:02:19.930 And the way to do that-- let's[br]see, I want to eliminate, I 0:02:19.930,0:02:22.210 feel like eliminating this y,[br]and I think you'll get 0:02:22.210,0:02:24.630 an intuition for how we[br]can do that later on. 0:02:24.630,0:02:26.620 And the way I'm going to do[br]that is I'm going to make 0:02:26.620,0:02:29.250 it so that when I had this[br]to this, they cancel out. 0:02:29.250,0:02:31.340 Well, they don't cancel out[br]right now, so I have to 0:02:31.340,0:02:34.380 multiply this bottom equation[br]by 4, and I think it'll be 0:02:34.380,0:02:35.520 obvious why I'm doing it. 0:02:35.520,0:02:37.810 So let's multiply this[br]bottom equation by 4. 0:02:37.810,0:02:50.820 And I get 16x plus 4y is equal[br]to 40 plus 32 minus 72. 0:02:50.820,0:02:51.130 Right? 0:02:51.130,0:02:53.950 All I did is I multiplied[br]both sides of the 0:02:53.950,0:02:55.620 equation by 4, right? 0:02:55.620,0:02:57.210 And you have to multiply[br]every term because 0:02:57.210,0:02:59.500 it's the distributive[br]property on both sides. 0:02:59.500,0:03:01.050 Whatever you do to one side[br]you have to do to the other. 0:03:01.050,0:03:03.300 Let me rewrite top[br]equation again. 0:03:03.300,0:03:05.230 And I'll write in the same[br]color so we can keep 0:03:05.230,0:03:06.340 track of things. 0:03:06.340,0:03:13.360 9x minus 4y is[br]equal to minus 78. 0:03:13.360,0:03:18.580 OK, well now, if we were to add[br]these two equations, when you 0:03:18.580,0:03:20.430 add equations, you just add[br]the left side and you 0:03:20.430,0:03:22.270 add the right side. 0:03:22.270,0:03:25.440 Well when you add, you[br]have 16x plus 9x. 0:03:25.440,0:03:28.590 Well that equals 25x. 0:03:28.590,0:03:28.950 Right? 0:03:28.950,0:03:31.450 16 plus 9. 0:03:31.450,0:03:34.910 4y minus 4, that just equals 0. 0:03:34.910,0:03:43.680 So that's plus 0 equals, and[br]then we have minus 72 minus 78. 0:03:43.680,0:03:51.490 So, let's see that's minus[br]150, minus 150, right? 0:03:51.490,0:03:53.060 Just adding them all together. 0:03:53.060,0:03:58.820 So we have 25x equals 150. 0:03:58.820,0:04:03.420 Well, we could just divide both[br]sides by 25 or multiply both 0:04:03.420,0:04:05.380 sides by 1/25, it's[br]the same thing. 0:04:05.380,0:04:08.470 And you get x equals--[br]that's a negative 150 0:04:08.470,0:04:11.500 --x equals minus 6. 0:04:11.500,0:04:14.870 There we solved[br]the x-coordinate. 0:04:14.870,0:04:16.950 Now to solve the y-coordinate[br]we can just use either one of 0:04:16.950,0:04:18.500 these equations up at top. 0:04:18.500,0:04:20.810 So let's use this one,[br]it seems a little bit, 0:04:20.810,0:04:23.020 marginally simpler. 0:04:23.020,0:04:26.090 So we just substitute the x[br]back in there and we get 0:04:26.090,0:04:34.716 4 time minus 6 plus y[br]is equal to minus 18. 0:04:34.716,0:04:35.730 Go up here. 0:04:35.730,0:04:42.565 4 times minus 6 we get minus 24[br]plus y is equal to minus 18. 0:04:42.565,0:04:47.406 And then get y is[br]equal to 24 minus 18. 0:04:47.406,0:04:50.510 So y is equal to 6. 0:04:50.510,0:04:54.100 So these two lines or these two[br]equations, you could even say, 0:04:54.100,0:05:00.300 intersect at the point x is[br]m inus six and y is plus 6. 0:05:00.300,0:05:02.520 So they actually intersect[br]someplace around here instead. 0:05:02.520,0:05:05.640 I drew these, the line probably[br]look something more like that. 0:05:05.640,0:05:06.950 But that's pretty cool, no? 0:05:06.950,0:05:11.830 We actually solved for two[br]variables using two equations. 0:05:11.830,0:05:12.640 Let's see how much time I have. 0:05:12.640,0:05:14.470 I think we have enough time[br]to do another problem. 0:05:14.470,0:05:20.200 [br]105[br]00:05:20,2 --> 00:05:23,02[br]So let's say I had the points--[br]and I'm going to write them in 0:05:23.020,0:05:32.940 two different colors again[br]--minus 7x minus 4y equals 9, 0:05:32.940,0:05:39.150 and then the second equation is[br]going to be x plus 0:05:39.150,0:05:42.460 2y is equal to 3. 0:05:42.460,0:05:45.140 Now if I were doing this as[br]fast as possible, I'd probably 0:05:45.140,0:05:47.990 multiply this equation times 7[br]and it would automatically 0:05:47.990,0:05:49.020 cancel out. 0:05:49.020,0:05:49.850 But that's easy way. 0:05:49.850,0:05:51.290 I'm going to show you that[br]sometimes you might have to 0:05:51.290,0:05:54.780 multiply both equations--[br]actually, not in this case. 0:05:54.780,0:05:56.800 Actually let's just do it[br]the fast way real fast. 0:05:56.800,0:05:59.380 So let's multiply this[br]bottom equation by 7. 0:05:59.380,0:06:00.830 And the whole reason why I want[br]to the, multiply it with 7, 0:06:00.830,0:06:03.440 because I want this to[br]cancel out with this. 0:06:03.440,0:06:10.150 If you multiply it by 7 you get[br]7x plus 14y is equal to 21. 0:06:10.150,0:06:12.930 Let's write that first[br]equation down again. 0:06:12.930,0:06:19.065 Minus 7x minus 4y[br]is equal to 9. 0:06:19.065,0:06:20.330 Now we just add. 0:06:20.330,0:06:24.260 This is a positive 7x, it just[br]always looks like a negative. 0:06:24.260,0:06:25.900 OK, so that's 0. 0:06:25.900,0:06:32.460 14 minus 4y plus 10y[br]is equal to 30. 0:06:32.460,0:06:34.750 y is equal to 3. 0:06:34.750,0:06:36.350 Now we just substitute back[br]into either equation, 0:06:36.350,0:06:37.980 lets do that one. 0:06:37.980,0:06:42.110 x plus 2 times y, 2 times 3. 0:06:42.110,0:06:43.880 x plus 6 equals 3. 0:06:43.880,0:06:45.900 We get x equals negative 3. 0:06:45.900,0:06:48.470 That one was super easy. 0:06:48.470,0:06:49.550 The intercept. 0:06:49.550,0:06:51.210 Hope I didn't do it to fast. 0:06:51.210,0:06:54.430 Well, you can pause it and[br]watch it again if you have. 0:06:54.430,0:07:00.270 OK, so these two lines[br]intersect at the point 0:07:00.270,0:07:03.182 negative 3 comma 3. 0:07:03.182,0:07:04.250 Let's do one more. 0:07:04.250,0:07:07.456 [br]140[br]00:07:07,456 --> 00:07:10,71[br]Hope this one's harder. 0:07:10.710,0:07:11.510 I think it will. 0:07:11.510,0:07:20.300 OK, negative 3x minus[br]9y is equal to 66. 0:07:20.300,0:07:27.200 We have minus 7x plus 4y[br]is equal to minus 71. 0:07:27.200,0:07:28.370 So here it's not obvious. 0:07:28.370,0:07:31.540 What we have to do is, let's[br]say we want to cancel 0:07:31.540,0:07:33.980 out the y's first. 0:07:33.980,0:07:36.500 What we do is we try to make[br]both of them equal to the least 0:07:36.500,0:07:38.660 common multiple of 9 and 4. 0:07:38.660,0:07:43.340 So, if we multiply the top[br]equation by 4 we get-- 0:07:43.340,0:07:44.520 I'll do it right here. 0:07:44.520,0:07:45.870 Let's multiply it by 4. 0:07:45.870,0:07:47.960 Times 4. 0:07:47.960,0:07:59.200 We'll get minus 12x minus[br]36y is equal to 4 times 0:07:59.200,0:08:05.400 240 plus 24 is 264. 0:08:05.400,0:08:06.930 Right, I hope that's right. 0:08:06.930,0:08:09.220 We multiply the second[br]equation by 9. 0:08:09.220,0:08:25.420 So it's minus 63x plus 36y is[br]equal to, let's see, 639. 0:08:25.420,0:08:26.030 Big numbers. 0:08:26.030,0:08:29.350 639. 0:08:29.350,0:08:31.540 OK, now we add the[br]two equations. 0:08:31.540,0:08:43.570 Minus 12 minus 63 thats minus[br]75x-- these cancel out --equals 0:08:43.570,0:08:50.130 264, let's see what's[br]639 minus 264. 0:08:50.130,0:08:51.160 See I do this in real time. 0:08:51.160,0:08:55.100 I don't use some kind of[br]solution manual or something. 0:08:55.100,0:08:59.710 13 and 5, 70. 0:08:59.710,0:09:02.260 I don't know if I'm[br]right, but we'll see. 0:09:02.260,0:09:06.360 Since it's actually the[br]negative 639, this is minus 0:09:06.360,0:09:12.440 375, and I know that seventy[br]five goes into 300 4 0:09:12.440,0:09:16.450 times, so x is equal to 5. 0:09:16.450,0:09:19.515 75 times 5 is 375. 0:09:19.515,0:09:22.460 We just divided[br]both sides by 75. 0:09:22.460,0:09:25.367 So if x is 5 we just substitute[br]it back into-- let's 0:09:25.367,0:09:27.890 use this equation. 0:09:27.890,0:09:36.380 So we get minus 3 times 5[br]minus 9y is equal to 66. 0:09:36.380,0:09:41.920 We get minus 15[br]minus 9y equals 66. 0:09:41.920,0:09:45.880 Minus 9y is equal to 81. 0:09:45.880,0:09:49.840 And then we get y is[br]equal to minus 9. 0:09:49.840,0:09:53.530 So the answer is[br]5 comma minus 9. 0:09:53.530,0:09:55.530 I think you're ready to do some[br]systems of equations now. 0:09:55.530,0:09:57.090 Have Fun.