# Thread: system of equations alg II

1. ## system of equations alg II

Once again, easy but i can't remember...sorry for another question but i think its my last one...

Solve the system of equations and check your answer(s). You may use any method to solve.

2x^2 - y = 8
7x + y = -4

I tried it this way:

y = -7x - 4

2x^2 - (-7x - 4) = 8

2x^2 + 7x + 4 = 8

2x^2 + 7x - 4 = 0

(2x -1) (x + 4) = 0

x = 1/2, -4

Now, when I plug in both values for x (because there were 2 values) seperate times i get values for y that don't check out when I check both values for x and y. HELP!!! Thank you so much!

2. Hello,

Originally Posted by eah1010
Once again, easy but i can't remember...sorry for another question but i think its my last one...

Solve the system of equations and check your answer(s). You may use any method to solve.

2x^2 - y = 8
7x + y = -4

I tried it this way:

y = -7x - 4

2x^2 - (-7x - 4) = 8

2x^2 + 7x + 4 = 8

2x^2 + 7x - 4 = 0

(2x -1) (x + 4) = 0

x = 1/2, -4

Now, when I plug in both values for x (because there were 2 values) seperate times i get values for y that don't check out when I check both values for x and y. HELP!!! Thank you so much!
This is normal, because there are a couple of solutions !

For the value x=1/2, y=-15/2.

For the value x=-4, y=24.

So the set of solutions is :

$S=\left\{\left(\frac 12, -\frac{15}{2}\right) ~,~ (-4,24)\right\}$

Is it better this way ?

3. Well, it's probably my math because I am extremely bad at making little mistakes, but when i plug 1/2 into the 1st equation i got y=4.5 and when I plug in -4 as x i get y=36. Im just confused on the whole thing now! Haha! any help?

4. Originally Posted by eah1010
Well, it's probably my math because I am extremely bad at making little mistakes, but when i plug 1/2 into the 1st equation i got y=4.5 and when I plug in -4 as x i get y=36. Im just confused on the whole thing now! Haha! any help?
y=-7x-4

If x=1/2
y=-7/2-4=-15/2=-7.5

If x=-4
y=-7*(-4)-4=28-4=24