subsitution, elimination, and foil

HispanicIcePrincess

New member
Joined
Jun 12, 2007
Messages
1
Hello! I was just wondering, when using the elimination method (multiplication/division), do you add when the signs are the same or different? For instance, in the problem,

3x+8y=1
7x+4y=17,

after multiplying the top equation by seven and the bottom one by three to get

21x+56y=7
21x+12y=51

subtraction rather than addition is used, being that the two twenty-ones share the same sign, right?

Is (3,-1) the correct answer for this problem?

I'm sorry for bombarding you with so many questions and such inconvenience, but I'd greatly appreciate it if I received an accurate reply to my inquiries by tomorrow night. The earlier the better, but don't stress on it too much. It's just that I have to take my math final on Thursday =( Therefore, it would be highly appreciated to have an answer as soon as possible =) Well, thanks for your cooperation. I'm looking forward to hearing from you!
 

jonboy

Full Member
Joined
Jun 8, 2006
Messages
547
There's an easier way. Your way uses big numbers. Smaller the number it's the easier right (unless fractions) ?

...3x + 8y = 1 [1]
...7x + 4y = 17 [2]

Multiply [2] by 2, name it [3]:

...14x + 8y = 34 [3]
...3x + 8y = 1 [1]

Subtract like terms in [3] and [1]: 14x - 3x + 8y - 8y = 34 - 1

...11x = 33

...x = 3

Plus x in to solve for y:...3(3) + 8y = 1

...9 + 8y = 1

...y = -1

So you are correct! Great work!
 

jwpaine

Full Member
Joined
Mar 10, 2007
Messages
723
Check by plugging in your answer of x and y into both equations. IF the equations are true, than your answer is correct.
 
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