systems of equations

[TheSoccer16x]

i love how this shows exactly what my math teacher is teaching me tomorrow..

[XmayaXeliasX]

okay. soo.. what about the 40???

[MrCIickHere]

I'll agreed. These video really helpful

[activatupc]

thanks i'm from Mexico but in my country doesn't explain like you :D sorry for my bad english (Y)

[nitrogif]

would like to see this remade with colors etc.

[Happyascanbe1]

In the last example you have multiplied the first equation with 4 to get -36y and 36y and you've got: -12x -36y = 264. Now, i just noticed that if you multiplied it with 7 instead to get -21x and 21 x, you would have gotten: -21x - 63y =462 and i couldn't help but notice the cool inversion of digits. How do you explain that?

[roboroborobodancer]

Made perfect sense after first question. Really, thanks a lot!

[theicesarcontreras]

I would usually understand your videos but this was the worst.

[moihancalsar]

5 ppl failed math

[klevdav]

Wtf!!!!! Is this

[HyperNovaTornado]

How do you find the intersection of 2 curves without graphing?

[Devestato72]

who cares?

[michaelsantiago1000]

YOU SUCK AT LIFE

[michaelsantiago1000]

YOUR A TERRIBLE TEACHER 

[iWriteUinMyDeathNote]

qgaaaaaa

[Johnnylightning93]

Where did he get y=81 in the 3rd problem??

[simplayaj]

@Johnnylightning93 add 15 to both sides

[Johnnylightning93]

@simplayaj thanks

[UKWildcatsFTW2012]

Your videos help a lot. Thank you so much Mr. Khan!

[jesikasamudio]

Thank you your video help my daughter!

[TheAuthenticStar]

this is really helpful but how do you know what to multiply the equation by

for example: you multiplied x+2y=3 by 7

i don't understand that part .

[ChanceMyHeart]

I use to just flat guess by bubble in a guess. Now from watching this video it made me stop guessing an gave me better chance of getting it right. I truly was simply lost in math yet some how this man is really getting it plant in my brain. He is amazing teacher.

[koolvhin]

Is (-6,6) found in the 4th or 2nd quadrant?

[M0ey6654]

Thank you, Mr. Khan! I am ready to do some equations now, and I WILL have Fun!

[Jadore780]

i missed this class and watching your video is soo helpful ! iwish i watched this a few days before because tonight is my final exam!! ah!!

[vickiormindyb]

A teacher told me I'm a hopeless case and would never work "beyond a sixth grade Math level."

Well, my stubbornness refused to believe that would happen b/c of a drunk driver hitting my car headfirst (which is how I got the left-side traumatic brain injury), and I found Khan Academy through my obstinate search for evidence to defy her.

I never knew I'd actually find it, but I did, and I wish I could find her and tell her so. Even better, show her.

[TheAlexa017]

what if both equations (top and bottom) are like this: -9x - 4y = -24, -4x - 9y =11?

[thepapargangstar]

Mr. Kahn i have the problem 0.5p=p+5 can you help

[slipperyjack007]

@thepapargangstar its easy

"p" on its own is the same as 1p.

so, 0.5p-1p=5

-0.5p=5

p=5/-0.5

so p=-10

[IBlackI1]

YES THIS IS THE PROBLEM I WAS STRUGGLING WITH!

[EloiseeR]

i'd never heard of systems of equations but then i realised that it was because in the UK we call it 'simultaneous equations'

[megankittty]

@WeAreStarStuff51 ohhhh i see. ty

[maxmadsens]

system of linear eq.

[megankittty]

how'd you get the 16x???

i'm confuzzzled!

[WeAreStarStuff51]

@megankittty You multiply 4x by 4 so, (4x)(4)=16x. This is all done so you can do (y)(4)=4y so the two 4y's cancel

[hobomnky]

my teacher is useless, he cant speak english, and hes from china ... idk if this is normal for him but on the first day he wrote all of the quadratics unite on the board and asumed we would understand, in addition to he didnt realise we didnt get wat he was saying, nor did he understand our questions, last year i was top 94% this year im at 78.. which is clearly not my fault!

[AxL456]

do you have a video for systems of nonlinear equations??

[KRBfoevah]

hey??where'd you get the 16x???

[PrinceFX]

@KRBfoevah multiply 4x by 4, that is (4x)*(4) or 4(4x)

[kyea67]

i believe that the two equations goes as follows

x=12 percent cadi

y=20 percent solution

.12x+.20y=200(.18)

x+y=200

you should be bale to get your answer from there

[Kholberg94]

A chemistry lab needs to make 200 gallons of an 18% acid solution by mixing a 12% cadi solution with a 20% solution. How many gallons of each solutions are needed?

please help

[Heatherinski]

You're awesome! Thanks so much for all you do!

[kyrashea21]

you rock

[DeViLiN888]

Mr. Khan could you make a video on how to solve a 3 variable system?

[smalljonny]

@DeViLiN888 I'll solve a 3 variable system. It involves sub a few equations into one of the three equations to find the 3 unknowns x,y and z.

[dmcarefuldriver]

@DeViLiN888 You have to set up 2 equations without one of the variables by eliminating it the same way. Then the rest is the same. Example: x+y+z=6 2x-y+3z=9 -x+2y+2z=9 Add the first 2 equations to eliminate y. So you get: 3x+4z=15 Then multiply the second equation by 2 and add it to the 3rd equation to create another equation without y. 3x+8z=27 From here, we subtract to see that 4z=12, so z=3. Substitute into 3x+4z=15 > 3x +12=15 > x=1. Now, if x+y+z=6, then 1+y+3=6, so y=2. (x,y,z)=(1,2,3)

[DeViLiN888]

@dmcarefuldriver Thank you very much! This helps a lot!

[dmcarefuldriver]

@DeViLiN888 no problem

[greenpondalgae]

Hahaha! You make this so easy!!! Thanks, these hep a lot. :)

-GreenPondAlgae

[lukeman3323]

This is a good explanation of the topic. The only serious thing that I see is on your site. The lessons that I am currently doing in school are substitution, elimination, and graphing to find this. Based on how much of this that you can do, I'm sure that you could make a video on them. That would have helped me more than looking on the site, rephrasing my search many times, and eventually finding it.

Thanks for the video! It was a big help

~Lukeman3323

[rydawgpwnsu]

its ok

[Unknown1101]

You're genius!

[thenightbus]

omg thank you soo much!!

[ceomeye]

Xcellent

[sharkbait2712]

I attempted the first problem before watching the explanation. I subtracted 4x from the second equation, leaving me with y = -4x - 18. Then I plugged this y value into the other equation. Upon checking my answers, they came out right, but my x and y values were not (-6, 6); they were (-6/25, -17 &1/25). Like I said, upon plugging them back in, it all came out right, even though they're messy numbers. I just want to know whether this method was correct. Thank you.

[HumidCrispyCat]

dude. Thank you.

[remusikmoro]

thank you

[nafizkarim]

very nice! Sometimes it would be easier if you solved for x or y on one equation and plug it in to the second equation. No need to multiply then add.

[david0aloha]

It's two different methods, subsitution, and addition. Both work great, but addition is useful later on with larger systems and matrices.

[TENspeaks]

This is one of the only adequate explainations I've heard thus far for solving simeltaneous equations by addition. Great stuff!!!

[riskpuzzel]

wow thanks so much i subscribed! because of u i did well on my final

[LendaryMel]

At 8:40, What would happen if I was working with a similar problem and I still had a y variable? For example, both the equations have positive signs.

[SupaMANGO44]

Thank you man. You duh best

[Eatmiish0rtSsxD]

Do you always have to add to get either x or y alone? or can you also subtract to get either x or y alone.

[lawtonlynx12]

i think this is pointless...how does -78 -18 = -72????

great video if u ask me!!! lol NOT

[heymavis]

-18 multiplied by 4 = -72

I don't think anyone thinks -78-18 = -72

[lccovegirl]

i have a test on graphing these. so i was wondering, after i find the two coordinates i just graph it there?

[jennahlane12]

My math teacher showed us how to do this today and wants us to take an exam on it tomorrow. And i still dont know how to do it.....

[Winsunn307]

Very helpful, thanks.

[dirtybill94]

i am working on solving systems of equations by the simple elimination method

how do i do that?

[killertont]

how do you get 72 from?

[imjustkiddin]

Thank you so much, my current alg II teacher isn't really teaching us well and this is a really good substitution.

[HighLarious]

Excellent job -- so helpful. Thanks again!

[Nandine2]

thank you

[danjandris]

Thanks, your videos are helpful.