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Simultaneous Equations

Solve the simultaneous equations

x - 3y + 7 =0
X^2 + 2xy- y^2 =7

I tried rearranging the top equation and then substituting it in but my answer came out as 14y^2 -14y +49 = 7 which I don't think is right and I need to know what y=

... Could someone tell me how to answer this or maybe even give hints to send me in the right direction?

Reply 1

__Adam__

I get 14y^2 - 56y +49 = 7

When you expanded (3y-7)^2 did you expand it correctly?

Reply 2

Squidgy

Original post by __Adam__

I get 14y^2 - 56y +49 = 7

When you expanded (3y-7)^2 did you expand it correctly?

I expanded that to 9y^2 + 49.

I can't see where you got the -56y from

Reply 3

davros

Original post by Squidgy

I expanded that to 9y^2 + 49.

I can't see where you got the -56y from

What's happened to your "cross term"? (a+b)^2 = a^2 + 2ab + b^2 :smile:

Reply 4

__Adam__

Original post by Squidgy

I expanded that to 9y^2 + 49.

I can't see where you got the -56y from

(3y-7)^2 = 9y^2 - 21y -21y + 49

Remember to multiply the terms inside the bracket together as well.

(a+b)^2 = a^2 + 2ab + b^2

Reply 5

Blobar

Wrong expansion thats why, remember its (3y-7)(3y-7)

So you get 9y^2-42y+49

Reply 6

Squidgy

Original post by __Adam__

(3y-7)^2 = 9y^2 - 21y -21y + 49

Remember to multiply the terms inside the bracket together as well.

(a+b)^2 = a^2 + 2ab + b^2

Ahh yeah that's easy, I just home and haven't slept for like 26 hours, so my brain isn't functioning ... Shouldn't have left the work till the last day XD

Thank you :smile: