When using the elimination method (addition and subtraction)
When do you know when to add or subtract?

Question

When using the elimination method (addition and subtraction)
When do you know when to add or subtract?

amistre64 · Answer

just add at all times ...

anonymous · Answer

OK So don't bother about subtracting?

amistre64 · Answer

$$5-3~=~5+(-3)$$

amistre64 · Answer

subtraction isnt really an algebraic operation, its just a shortcut notation.

anonymous · Answer

Thanks! My book is confusing me with the 
"Use addition and subtraction, for the elimination method"
I think they purposelessly make it confusing.

amistre64 · Answer

the idea is simply, to eliminate one of the variables, you need them to be opposite in sign; and then combining like "parts" eliminates that variable

amistre64 · Answer

3x + 2y = 4
  x + 5y = -10

if we multiply the 2nd one by -3 we can combine to get rid of the xs

 3x + 2y =   4
-3x -15y = 30
---------------
0x -13y = 34

amistre64 · Answer

once you know a y value ... you dont need to eliminate again. just plug in y=___ into either equation to solve for x

anonymous · Answer

Then you can solve by dividing 34/13!
No subtraction necessary :D
I know I was on to something when I never understood subtraction ;P

amistre64 · Answer

:)  good luck