The Student Room Group

C1 differentiation

Morning everyone,

I've got a question and my answer has revealed that I've got the wrong idea.

'Determine the two pints on the curve with equation y=(x2+3)(x5)y=(x^2+3)(x-5) where the gradient is equal to -5.

So this is what I did. I simplified the equation, differentiated it, then carried -5 over to the other side so I had a quadratic. I could factorise it so I put it through the quadratic equation. This must have been the wrong thing to do though as the answer is wrong :frown:
Reply 1
What answers did you get
Reply 2
what was the correct answer
Reply 3
Original post by TenOfThem
What answers did you get



I got 5±313 \frac{5 \pm \sqrt{31}}{3} for x. The correct answers for x were 2 or 1&(1/3)
(edited 11 years ago)
Reply 4
Original post by gavinlee
Morning everyone,

I've got a question and my answer has revealed that I've got the wrong idea.

'Determine the two pints on the curve with equation y=(x2+3)(x5)y=(x^2+3)(x-5) where the gradient is equal to -5.

So this is what I did. I simplified the equation, differentiated it, then carried -5 over to the other side so I had a quadratic. I could factorise it so I put it through the quadratic equation. This must have been the wrong thing to do though as the answer is wrong :frown:


Is this right?

y=(x2+3)(x5)y=(x^2+3)(x-5) = y=x35x2+3x15y=x^3-5x^2+3x-15
dydx=3x210x+3\frac{dy}{dx}= 3x^2-10x+3
when dydx=5\frac{dy}{dx} = -5 then 3x210x+8=03x^2-10x+8=0
which equals (3x4)(x2)(3x-4)(x-2) so x=43or2\frac{4}{3} or 2
Reply 5
What was your quadratic ... the book answers are correct


Then again ... do not worry ... Rachel has taken away your chance to learn by posting the full answer
(edited 11 years ago)
Reply 6
Original post by rachel0593
Is this right?



There is a good reason that the forum frowns on full solutions

Gavin needed to learn where he was going wrong on this question ... he needed to unpick his misconceptions
Reply 7
Original post by TenOfThem
There is a good reason that the forum frowns on full solutions

Gavin needed to learn where he was going wrong on this question ... he needed to unpick his misconceptions


Since when does that work?, learn all the marks schemes and apply your knowledge is the best way to get the top marks.
Reply 8
Original post by Alex Dom
Since when does that work?, learn all the marks schemes and apply your knowledge is the best way to get the top marks.


My mistake

Clearly looking at other people's answers/the mark scheme is the way to succeed in maths exams

How stupid am I

Where on earth did I get my foolish view that understanding the material and the question was the way forward
Reply 9
Yeah I tried it again and got the right answers :smile:
Original post by gavinlee
Yeah I tried it again and got the right answers :smile:


ok :smile:
Reply 11
Sorry I though that by showing how I got to the answer he would see where he went wrong and could apply it to another question.
Reply 12
Original post by TenOfThem
My mistake

Clearly looking at other people's answers/the mark scheme is the way to succeed in maths exams

How stupid am I

Where on earth did I get my foolish view that understanding the material and the question was the way forward


Just talking out of experience. He might learn that way too.
Original post by rachel0593
Sorry I though that by showing how I got to the answer he would see where he went wrong and could apply it to another question.


In that case ... sorry I snarled at you
Original post by Alex Dom
Just talking out of experience. He might learn that way too.


Durrrrrrr

Again ... my stupidity ... of course with your vast experience you know what the best way for people to learn is
Original post by gavinlee
I got 5±313 \frac{5 \pm \sqrt{31}}{3} for x. The correct answers for x were 2 or 1&(1/3)


no no no i think you havent put the correct sign for -5
thats the only way you get your answer
(edited 11 years ago)
Reply 16
Original post by Alex Dom
Since when does that work?, learn all the marks schemes and apply your knowledge is the best way to get the top marks.


What are top marks in your opinion?

I hardly do 1-2 past paper, i just have a quick view of the mark scheme of those papers, so does this means that i can never get top marks? :frown:


Original post by Alex Dom
Just talking out of experience. He might learn that way too.


You look to be very experienced, can you give more advice on preparing maths?

If someone follows your advice for A-Level maths, and then if he has to give STEP/AEA, than what shall he do to prepare?
Learn all the STEP/AEA mark schemes?
Reply 17
Original post by TenOfThem
Durrrrrrr

Again ... my stupidity ... of course with your vast experience you know what the best way for people to learn is


:lol:

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