The Student Room Group

Formula Rearrange got me confused?

Take a look at the pic below.
This is the working to workout the equation of a straight line when all you are given is the gradient and any one coordinate.

I understand the formula down until the last line and then am becoming confused.

I totally see how the formula has been rearranged, it has minused the (2y + 4) from both sides. I worked the other way and minuses the (x - 3) from both side which gave me a totally different final answer of '2y + 7x - x = 0'.

Why has the text book done it the other way? Who is correct? Or would both receive full marks in an exam???

1411287661571.jpg1411287661571.jpg
2y+4=x-3

Take (x-3) from both sides you get
2y+4-(-3)-x=0
2y+4+3-x=0
2y+7-x=0
2y-x+7=0

If you want to put it so 'x' is the first term you would write it as -x+2y+7=0. If you want the 'x' to be positive just reverse all the signs so you get x-2y-7=0
Original post by lewif002
Take a look at the pic below.
This is the working to workout the equation of a straight line when all you are given is the gradient and any one coordinate.

I understand the formula down until the last line and then am becoming confused.

I totally see how the formula has been rearranged, it has minused the (2y + 4) from both sides. I worked the other way and minuses the (x - 3) from both side which gave me a totally different final answer of '2y + 7x - x = 0'.

Why has the text book done it the other way? Who is correct? Or would both receive full marks in an exam???




You have written 2y + 7x - x but I assume you mean 2y + 7 -x = 0

The answers are the same (multiply each side by -1) but as the other poster says just change to sign of each term.

In exams, in my experience, they ask for the answer to be in the form ax + by +c = 0 format so you could write -x +2y + 7 = 0 but people often, if possible, seem to start their expressions with a positive first term.

I'm not an expert, but having done C1 in the recent past and looked at mark schemes for AQA, that's how it seems to be. Sometimes the mark schemes say something like "or any equivalent". I'd check it out with your exam board and teacher but I hope what I've said has been of some use.




Reply 3
Original post by maggiehodgson
You have written 2y + 7x - x but I assume you mean 2y + 7 -x = 0

The answers are the same (multiply each side by -1) but as the other poster says just change to sign of each term.

In exams, in my experience, they ask for the answer to be in the form ax + by +c = 0 format so you could write -x +2y + 7 = 0 but people often, if possible, seem to start their expressions with a positive first term.

I'm not an expert, but having done C1 in the recent past and looked at mark schemes for AQA, that's how it seems to be. Sometimes the mark schemes say something like "or any equivalent". I'd check it out with your exam board and teacher but I hope what I've said has been of some use.







Thank-you! :smile:

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