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# help with simultaneous equation

1. Hi so the simultaneous equations are x^2-2xy-y^2=7
x+y=1
I rearranged x+y=1 to get x=1-y
I substituted into the first equation
which i got (1-y)^2 -2y(1-y)-y^2=7
after expanding the brackets and simplifying i got -6-3y+y^2=0
I feel like i have done something wrong as it cant be factorised and I went onto desmos and putting this into a quadratic equation gives me wrong solutions. Have I went wrong expannding brackets?
need help thank you.
2. (Original post by 5h3bi)
Hi so the simultaneous equations are x^2-2xy-y^2=7
x+y=1
I rearranged x+y=1 to get x=1-y
I substituted into the first equation
which i got (1-y)^2 -2y(1-y)-y^2=7
after expanding the brackets and simplifying i got -6-3y+y^2=0
I feel like i have done something wrong as it cant be factorised and I went onto desmos and putting this into a quadratic equation gives me wrong solutions. Have I went wrong expannding brackets?
need help thank you.
Yeah you expanded wrong, try expanding again more carefully. Post a picture of your working out of the expansion if it still doesn't work.
3. (Original post by Zacken)
Yeah you expanded wrong, try expanding again more carefully. Post a picture of your working out of the expansion if it still doesn't work.
Now i seem to be getting -6+y+y^2=0
seems to still be wrong
4. (Original post by 5h3bi)
Now i seem to be getting y^2+y-6=0
seems to still be wrong
Like I said, post a picture of your working out for the expansion or type it up.
5. (Original post by Zacken)
Like I said, post a picture of your working out for the expansion or type it up.
(1-y)^2-2y(1-y)-y^2=7
(1-y) (1-y) when i expand this i get 1-y-y+y as 1*1 is 1, 1* -y is -y and -y again and -y*-y is +y
and when i expand 2y(1-y)
i get 2y-2y^2
so this gives me
1-y-y+y - 2y-2y^2-y^2=7
collecting like terms I get y-2y-y^2=7
subtracting 7 to get
y-2y-y^2-7=0
i keep expanding and getting different answers?
6. (Original post by 5h3bi)
(1-y)^2-2y(1-y)-y^2=7
(1-y) (1-y) when i expand this i get 1-y-y+y as 1*1 is 1, 1* -y is -y and -y again and -y*-y is +y

No.

(Original post by 5h3bi)
.. - 2y-2y^2-y^2=7
Also no.
7. (Original post by 5h3bi)
(1-y)^2-2y(1-y)-y^2=7
(1-y) (1-y) when i expand this i get 1-y-y+y as 1*1 is 1, 1* -y is -y and -y again and -y*-y is +y
This bit is wrong, you get:
[...]
No full solutions.
9. (Original post by Zacken)
This bit is wrong, you get:
thanks it was due to my careless expanding of brackets, however I will now know for future lessons to carefully expand brackets. Thanks alot for helping Zacken and RDK games.
10. (Original post by 5h3bi)
thanks it was due to my careless expanding of brackets, however I will now know for future lessons to carefully expand brackets. Thanks alot for helping Zacken and RDK games.
You're welcome!
11. (Original post by Zacken;[url="tel:67647586")
67647586[/url]]No full solutions.
Haven't done simultaneous equations in ages haha

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