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Three equations, three unknowns

x+3y=2
-2x+3y+z=-1
y+z=-1

z=-1-y
x=2-3y

-2(2-3y)+3y+(-1-y)=-1
8y-5=-1
y=2

z=-1-2
z=-3

x=2-3(2)
x=-4

Can anyone see where I have gone wrong as my answers do not match the equations if plugged back in.
(edited 11 years ago)
Reply 1
Original post by Gorrilaz
x+3y=2
-2(2-3y)+3y+(1-y)=-1
8y-5=-1


You messed up here.

When you multiply out the brackets, you should have gotten:
-4 + 6y + 3y + 1 - y = -1

which simplifies to

8y - 3 = -1
Reply 2
Original post by FloydHead
You messed up here.

When you multiply out the brackets, you should have gotten:
-4 + 6y + 3y + 1 - y = -1

which simplifies to

8y - 3 = -1


No sorry it was actually-4+6y+3y-1-y=-1

As z=-1 -y I just write that part wrong.
Reply 3
Original post by Gorrilaz

8y-5=-1
y=2


Your mistake is above. 8*2 does not equal 4. :wink:
Reply 4
Original post by F1Addict
Your mistake is above. 8*2 does not equal 4. :wink:


Haha wow how did it take me so long to spot.
Thanks for your help
Reply 5
Original post by Gorrilaz
Haha wow how did it take me so long to spot.
Thanks for your help

You're welcome. :smile:

Sometimes the simplest things are hardest to spot. :smile:
Unless I'm badly mistaken, you can find all 3 variables from the first 3 equations given. Or is this one of those questions where you have to do it a given way? :s-smilie:
Reply 7
Original post by AtomSmasher
Unless I'm badly mistaken, you can find all 3 variables from the first 3 equations given. Or is this one of those questions where you have to do it a given way? :s-smilie:

The equations after the first 3 are working out, they're just rearranged to make x and z in terms of y. :wink:
Reply 8
I've been trying this another way but I just can't get to y = 2. Even when I put y = 2 to get x and z I get values that don't work out. Where am I going wrong?! Are y'all sure these equations are self consistent?????
Reply 9
Original post by fire sale
I've been trying this another way but I just can't get to y = 2. Even when I put y = 2 to get x and z I get values that don't work out. Where am I going wrong?! Are y'all sure these equations are self consistent?????


y=0.5 i did it wrong lol

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