Questions about the world of GRE Math from other sources and general math related questions.
baily106
Course Students
 
Posts: 49
Joined: Sat May 03, 2014 1:13 am
 

Adding & Subtracting Equations

by baily106 Fri Aug 08, 2014 6:22 am

Hello,

I've noticed that in certain problems you add the equations to find a solution and in certain problems you subtraction one equation from another to find the solution? How do you determine when to add the equations and when to subtract the equations?

Thanks.
tommywallach
Manhattan Prep Staff
 
Posts: 1917
Joined: Thu Mar 31, 2011 11:18 am
 

Re: Adding & Subtracting Equations

by tommywallach Sat Aug 09, 2014 10:19 pm

Hey Baily,

I'm not exactly sure what you mean, but I'll guess you're talking about the method of solving a linear system of two equations known as "elimination" (some people call it "combination," but the name "elimination" is better, because you're trying to eliminate a variable).

The goal is to eliminate a variable. So you'll just do whatever it takes to eliminate the variable.

For example, if you had these equations:

3x + 2y = 15
9x - 2y = 30

What action would eliminate a variable? Obviously, in this case, it would be adding, because 2y + (-2y) = 0.

In another example, if you had this:

5x + 7y = 22
3x + 7y = 30

What action would eliminate the variable here? Obviously, it would be subtraction, because 7y - 7y = 0.

So you just do it based on what will cause the correct result!

-t
baily106
Course Students
 
Posts: 49
Joined: Sat May 03, 2014 1:13 am
 

Re: Adding & Subtracting Equations

by baily106 Tue Aug 12, 2014 5:41 am

Ok, got it.
I wasn't sure if there was a rule, but as you explain it just sounds like follow logic :)
Thanks.
tommywallach
Manhattan Prep Staff
 
Posts: 1917
Joined: Thu Mar 31, 2011 11:18 am
 

Re: Adding & Subtracting Equations

by tommywallach Wed Aug 13, 2014 10:39 pm

Glad to help!

-t