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# FP2: Method of Differences - Where have I gone wrong? watch

1. (Original post by creativebuzz)
Attachment 462099462101

Ah I see!

Could you spot where I went wrong here?

Attachment 462099

(there are 2 attachments)

The question was |5x+a|<=|2x|
Your graphs aren't quite right - as you showed algebraically - both intersection occur at a negative value -a/something. Your graph should look like this (blue is 2x), ignore the values - I put a random value of a in - look at the position of intercepts and the general shape:

2. (Original post by Zacken)
Your graphs aren't quite right - as you showed algebraically - both intersection occur at a negative value -a/something. Your graph should look like this (blue is 2x), ignore the values - I put a random value of a in - look at the position of intercepts and the general shape:

Ah I get it!

I can't seem to spot where I went wrong in this question! Part A was to sketch the graph y=|x^2 - 6x +8| and 2y=3x-9 and Part B was to find where the 2|x^2 - 6x +8| and 3x-9 intersect! I got 4 values and I don't know why..

Attachment 462121462123Attachment 462121462123462140
Attached Images

3. (Original post by Zacken)
Your graphs aren't quite right - as you showed algebraically - both intersection occur at a negative value -a/something. Your graph should look like this (blue is 2x), ignore the values - I put a random value of a in - look at the position of intercepts and the general shape:

About that question, isn't it a bit strange because they advise that we draw the graph out first to see where the graphs intersect each other. Even-though that is what I did, because one of my graphs were a bit narrow than it should have been my graph misleads me into thinking that y=|5x+a| interest y=|2x| twice at y=2x and y=-2x leading to me setting up two equations 5x +a = 2x and 5x + a = -2x etc. However, your graph show that eventhough they intersect twice y=-2x intersects y=-5x-a and y=5x+a leading you to set up two different equations to me..
4. (Original post by creativebuzz)
About that question, isn't it a bit strange because they advise that we draw the graph out first to see where the graphs intersect each other. Even-though that is what I did, because one of my graphs were a bit narrow than it should have been my graph misleads me into thinking that y=|5x+a| interest y=|2x| twice at y=2x and y=-2x leading to me setting up two equations 5x +a = 2x and 5x + a = -2x etc. However, your graph show that eventhough they intersect twice y=-2x intersects y=-5x-a and y=5x+a leading you to set up two different equations to me..
As far as I can see, my two 'different' equations are equivalent to yours, just multiplied through by -1. Look below:

You came up with

which agrees with my

which agrees with my - they are literally the same in this case.

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