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# Implict Function P3 watch

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1. in edexcel p3, if they ask you to find gradient of an implicit function ( for example, x^2 +2xy+ y^2 + 16 = 0) at (3,16) or something like that, do you:

1. find dy/dx (by using implicit differentiation) first and then stick the values right away and then rearrange to make dy/dx the subject OR

2. rearrange FIRST and then stick the values in.

Im not sure which to go as they will produce different answers i think.
2. (Original post by Jubba)
in edexcel p3, if they ask you to find gradient of an implicit function ( for example, x^2 +2xy+ y^2 + 16 = 0) at (3,16) or something like that, do you:

1. find dy/dx (by using implicit differentiation) first and then stick the values right away and then rearrange to make dy/dx the subject OR

2. rearrange FIRST and then stick the values in.

Im not sure which to go as they will produce different answers i think.
You differentiate the whole expression with repsect to x and rearrange to find dy/dx and then sub the values of x and y into it.
3. They'll both give the same answer so it's a matter of preference really. If you're only finding one gradient then you may find putting values in first is easier/quicker, but if you need to find a couple, rearranging first will probably be quicker as you'll only need to rearrange once.
4. (Original post by Jubba)
in edexcel p3, if they ask you to find gradient of an implicit function ( for example, x^2 +2xy+ y^2 + 16 = 0) at (3,16) or something like that, do you:

1. find dy/dx (by using implicit differentiation) first and then stick the values right away and then rearrange to make dy/dx the subject OR

2. rearrange FIRST and then stick the values in.

Im not sure which to go as they will produce different answers i think.
i.e.

x^2 +2xy+ y^2 + 16 differentiates to:

2x+2x.dy/dx+2y+2y.dy/dx = 0
dy/dx = (-2x-2y)/(2x+2y) and sub x=3, y=16.
5. alright then, thanks 4 that.
6. I've just realised with this one if you rearrange with the symbols you get dy/dx = -1 which is independent of x or y so it's just a straight line
7. (Original post by Bezza)
I've just realised with this one if you rearrange with the symbols you get dy/dx = -1 which is independent of x or y so it's just a straight line
Yeah, should have seen it sooner, oh well, when's P3 again?
8. (Original post by Bhaal85)
Yeah, should have seen it sooner, oh well, when's P3 again?
January 2004, I got 100
9. (Original post by Bezza)
January 2004, I got 100
Fancy schmancy, show off. erm....I got an A at GCSE. lol.
10. (Original post by Bhaal85)
Fancy schmancy, show off. erm....I got an A at GCSE. lol.
I won't show off about my gcse maths. It was pretty good though.
11. (Original post by Bezza)
January 2004, I got 100
i aim for that this time

its on wednesday by the way
12. (Original post by kikzen)
i aim for that this time
You do that!!! I will be happy aslong as my exam actually achieves a grade (i.e. not a U)

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Updated: June 5, 2004
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