Really Hard Math Problem D:

puppyloverlauren • 27 November 2012 at 7:55 PM

I had so much stress in today's lesson! It is Solving Systems by Substitution!

Here is the problem:
( y-2x=3
<
( 2x-3y=21

Help?

@cat
@ancient_moonbeam
@dada_dragon
@jupiter_hollow
@bunnyshadow

iceyfira • 27 November 2012 at 7:58 PM

@puppyloverlauren


Im guessing x= 6.... and y=9? o-o Or the other way around. ^^;;;

puppyloverlauren • 27 November 2012 at 7:59 PM

@iceyfira

The back of the book says (-15/2, -12) D:

iceyfira • 27 November 2012 at 8:00 PM

@puppyloverlauren O-O Oy vey.

Erm.... I dunno then. XDDD

cotton • 27 November 2012 at 8:03 PM

@puppyloverlauren I used this when I had trouble with it at the beginning of the school year.
http://www.youtube.com/watch?v=KNwwu5wjcaA
It really helped me understand 😊 Hope it helps!

ancient_moonbeam • 27 November 2012 at 8:03 PM

y-2x=3 < 2x-3y=21 change to
y-2x=3 = 2x-3y=21 add the first equation
y-2x+2x-3y=21+3 combine like terms (the x's cancel)
-2y=24 divide off the -2
y=-12 put into either problem
-12-2x=3 algebra goes here
-2x=15
x=-7.5 check with the other one, use 12=y
2x+36=21 algebra
2x=-15
x=-7.5

There ya go

lily- • 27 November 2012 at 8:06 PM

@puppyloverlauren
y-2x=3

ancient_moonbeam • 27 November 2012 at 8:12 PM

Hey thats what I got! good to know I wasn't wrong

lily- • 27 November 2012 at 8:25 PM

@ancient_moonbeam
Same here, I saw your post after I posted. 😊

ancient_moonbeam • 27 November 2012 at 8:26 PM

Awesome @lily- ! Although it took me a little while to get it. Isn't this called the elimination method?

lily- • 27 November 2012 at 8:38 PM

@ancient_moonbeam
It is, isn't it.
Isn't substitution then when you make x= or y= to the rest of the equation and substitute it into the other one?

puppyloverlauren • 27 November 2012 at 8:38 PM

@ancient_moonbeam @lily-

Thank you so much! I get it now ^-^ sorry I was eating dinner

lily- • 27 November 2012 at 8:41 PM

@puppyloverlauren
You're welcome...though do you want the substitution method, since I ended up doing the elimination method?

puppyloverlauren • 27 November 2012 at 8:48 PM

@lily-

I don't know X3 I hate this homework... I might need help often ^^' like right now


2x+2y=2
-4x+4y=12

lily- • 27 November 2012 at 8:56 PM

@puppyloverlauren
I added the substitution method 😊
It's quite simple if you follow the same steps.
STEP 1:-
Make one of the equations y=.... or x=.....

STEP 2:-
Substitute whatever you get into your other equation instead of y or x (if you did y=... then you substitute it for y, if you did x=.... then you substitute it for x), so that you only have one unknown. (either y or x)

STEP 3:-
Simplify the equation and solve it so you find either y or x

STEP 4:-
Then substitute your answer into one of the equations you were given to get the other unknown. (so if you found x substitute that in to find y and if you found y you do the same to find x)

You can look at my method for your previous question with the steps so you can see how I did it to help you further.
I hope this helps 😊
If it doesn't I'll try to help you tomorrow, since now I need to go to sleep 😊

puppyloverlauren • 27 November 2012 at 9:01 PM

@lily- @ancient_moonbeam

I'm still confused D:

trish • 27 November 2012 at 9:09 PM

2x+2y=2
-4x+4y=12

Change the first equation into a y =....
So 2y= -2x+2

Divide the equation by 2 so it's y by itself.
2y (/2) =-2x+2 (/2)
So:
y=-1x +1

Plus the previous equation into your new one:
-4x+4(-1x+1)=12
So..
-4x+-4x+4=12
Combine like terms.
-8x+4=12
Subtract 4 from each side.
-8x=8
Divide =8 from both sides.
x=-1

Plug the x into the y function.
Look up Line 8: y=-1x+1
Plug x in.
y=-1(-1)+1
y=1+1
y=2

x=-1, y=2.
Somebody check for me?

twilight_raptor • 27 November 2012 at 9:13 PM

@puppyloverlauren will try to help 😸
You have 2 equations here and to begin you have to choose on of the unknown (x or y here) to solve for in an of the 2 equations of your choice.
1) y-2x=3
2) 2x-3y=21

I'll choose equation one and will solve for y:
1) y -2x +2x= 3 +2x
y= 3 +2x

The equation says that y is equal to (3 +2x). Now you can substitute this in place of y in the second equation:

2) 2x-3y=21

2x -3( 3 +2x) = 21 (Will name this equation 3)

using equation 3 you can solve for x since it is the only unknown there:

2x -3( 3 +2x) = 21
2x -9 -6x = 21
2x -6x = 21 +9
-4x = 30
x= -30/4
x= -7.5
Now that you know what x is you can substitute it in any of the equations to find y (I'll choose Eq #1):
1) y-2x=3
y -2(-7.5) = 3
y +15 = 3
y= 3 -15
y= -12

puppyloverlauren • 27 November 2012 at 9:13 PM

@trish

that's what I got but the book says (1/2, 1/2) T-T

trish • 27 November 2012 at 9:15 PM

@puppyloverlauren o-o Well then...

puppyloverlauren • 27 November 2012 at 9:18 PM

@trish
can you try this one then?

-x+y=4
-3x-2y=-7

trish • 27 November 2012 at 9:31 PM

-x+y=4
-3x-2y=-7

y=x+4

-3x-2(x+4)=-7
-3x-2x-8=-7
-5x=-15
x=3

y=3+4
y=7.

3,7.

Ta-da. Captain Algebra. o3o

puppyloverlauren • 27 November 2012 at 9:39 PM

@trish

the answer says (1,5) :/ this is so confusing!! DX

@cstarswirl
@skylark
@ems
@seaangel
@lynncrow

ems • 27 November 2012 at 9:41 PM

@puppyloverlauren

Wait what? xD That's really confusing.

I haven't done that stuff yet, but what do you need help with? I might be able to figure it out?

puppyloverlauren • 27 November 2012 at 9:41 PM

@ems

lol all of the problems are like these ones X3

duchess • 27 November 2012 at 9:43 PM

@puppyloverlauren Do you still need help? 😃

ems • 27 November 2012 at 9:43 PM

@puppyloverlauren

Just give me one. xD

It can't be that hard... Is it Pre-Algebra?

twilight_raptor • 27 November 2012 at 9:45 PM

@puppyloverlauren
1) -x+y=4
2) -3x-2y=-7

solve for y
1) y= 4 +x

substitute in place of y
2)
-3x-2y=-7
-3x -2( 4 +x) = -7
-3x -8 -2x = -7
-3x -8 +8 -2x = -7 +8
-3x -2x = 1
-5x =1
x= -(1/5)

substitute in place of x
1)
-x +y = 4
(1/5) +y = 4
y= 4-(1/5)
y= 3.8

x= -1/5 y=3.8

puppyloverlauren • 27 November 2012 at 9:45 PM

@duchess

Yes! Please! D:

@ems

No this is a algebra XD

ems • 27 November 2012 at 9:47 PM

@puppyloverlauren

JUST GIVE ME A PROBLEM. DX