simple algebra help please.

[cameron marical]

my brother has some math problems where he has to solve by graphing things like x+2y=10

3x+4y=8.

 

i know this is basic easy algebra, but i covered this stuff long ago and am more of a biology person anyways so i dont deal with much math at all anymore. can someone just give me the basics on how to do this?

thanks.

[samtheflash82]

It's called a system of equations. what you can do with something like that is solve for 1 variable (I'll use x) and then substitute in the second equation. I'll use your equations as an example.

 

the first equation

x+2y=10

x=10-2y

 

then we can substitute this value for x in the second equation.

3(10-2y)+4y=8

30-6y+4y=8

-2y=-22

y=11

 

then we can substitute for y back into the first equation and solve for x

x+2(11)=10

x+22=10

x=-12

y=11

[cameron marical]

ok. i was mixing this up with the little y=mx+b equation and some others. thanks alot man.

[samtheflash82]

no problem

[timo]

ok. i was mixing this up with the little y=mx+b equation and some others. thanks alot man.

Well, does you brother have to find a common solution for the two equations in any way or by graphing? Sam's way leads to a solution but not by graphing. For finding a solution by graphing (whyever you'd want that) you would indeed rewrite both equations in the form y=..., plot them both (possibly by formally replacing y with f(x), depending on the notation you are used to) and look for the intersection point(s).

[samtheflash82]

yes, there are many methods of solving a system of equations. you could also use the addition method where you set up the equations so that when you add the together in column form, one of the variables cancels and you are left with a solution to the other.

[the tree]

You do indeed need to rearrange the equation into the y=m*x+c format. The technique is of course less accurate, but you should probably follow the task given.

 

Ideally elimination would be the preferable technique, since unlike substitution it doesn't become brain fryingly difficult in more complicated situations. If you think one technique is better than the one your being asked for, do both.