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# C4 Implicit diffrentiation help please Watch

1. I cant seem to do this question, help appreciated...

its from elmwood paper C4 B, Question 1 , from free exam papers.com

a)find the gradient of the line with equation
(x-2)(y+5)=12

b)find the eqn of the normal at point (4,1)

when i did it i got
(-5-y)/(x-2) for part a

but the answer in mrskscheme is (y+5)/(2-x)
I dont understand why my signs are wrong, seem to make this mistake again and again, dont quite know why....
2. (Original post by hazu121)
I cant seem to do this question, help appreciated...

its from elmwood paper C4 B, Question 1 , from free exam papers.com

a)find the gradient of the line with equation
(x-2)(y-3)=12

b)find the eqn of the normal at point (4,1)

when i did it i got
(-5-y)/(x-2) for part a

but the answer in mrskscheme is (y+5)/(2-x)
I dont understand why my signs are wrong, seem to make this mistake again and again, dont quite know why....
(-5-y)(x-2) = (y+5)/(2-x), just factorise the -1 from the numerator and multiply it into the denominator.

However, (x-2)(y-3)=12 is not a line. Also, I have no clue where you're getting (y+5)/(2-x) from really.
3. (Original post by Swayum)
(-5-y)(x-2) = (y+5)/(2-x), just factorise the -1 from the numerator and multiply it into the denominator.

However, (x-2)(y-3)=12 is not a line. Also, I have no clue where you're getting (y+5)/(2-x) from really.
Hey thanks, for the speedy reply
Sorry I just realised that I made a mistake in the original post, should be (x-2)(y+5)=12
also, Im not sure what you mean
if you factorise -1 from the numerator it would be -1(y+5) /(x-2)
how do you then multiply it into the denominator, are you allowed to do that?
4. Yes, of course.

It works because -1 * -1 = 1, so rearranging gives -1 = 1/-1. And also -1 * 1 = -1, so rearranging gives -1 = -1/1. Put the two together and you get

-1 = 1/-1 = -1/1 (which means you can put the minus sign anywhere, numerator or denominator)
5. (Original post by Swayum)
Yes, of course.

It works because -1 * -1 = 1, so rearranging gives -1 = 1/-1. And also -1 * 1 = -1, so rearranging gives -1 = -1/1. Put the two together and you get

-1 = 1/-1 = -1/1 (which means you can put the minus sign anywhere, numerator or denominator)
Ok, I thought so...
but does that mean that either answer is accepted, though the markscheme one is seen as more acceptable as it is neater?
Sorry, it seems quite simple now
6. (Original post by hazu121)
Ok, I thought so...
but does that mean that either answer is accepted, though the markscheme one is seen as more acceptable as it is neater?
Sorry, it seems quite simple now
Yes, either would be fine.

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Updated: December 25, 2010
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