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# C4 Differentiation watch

1. Can anyone help me answer questions 16 and 17?
2. I could do parts A and B of Q16, but C and question 17 were tricky.
3. (Original post by jsk800)
Can anyone help me answer questions 16 and 17?
Differentiate the functions.
Plug in the relevant numbers
4. (Original post by jsk800)
I could do parts A and B of Q16, but C and question 17 were tricky.
Could you post where you've got to?

5. Done that for both questions. Now unsure about how to find the two coordinates. For 16 C, I understand that the equation will essentialy be X=constant, and therefore Dy/DX should be infinity, but the answer says I'm wrong so I don't know what to do. I understand that the formula is similar to that of a circle.
This is where I've made a mistake:
6. (Original post by Muttley79)
Could you post where you've got to?
^^^ Here
7. (Original post by jsk800)
^^^ Here
I have to say that I find your work very hard to follow.

Please write 'differentiating gives' rather than 'dy/dx'

Why do you swap everything around when there's no need to? Examiners will struggle to mark this.

I can't follow (c) at all - where does the line after the two equations come from?

In 17, I think you just need to carry on ....
8. (Original post by jsk800)
^^^ Here
16c.

For parallel to the y axis, yes, the denominator of dy/dx will be zero.

HOWEVER, the numerator won't.

Your two simultaneous equations will be:

Denominator is zero, and point lies on curve, so satisfies the equation of the curve.
9. (Original post by ghostwalker)
16c.

For parallel to the y axis, yes, the denominator of dy/dx will be zero.

HOWEVER, the numerator won't.

Your two simultaneous equations will be:

Denominator is zero, and point lies on curve, so satisfies the equation of the curve.
Okay so just 2y-x-1 = 0. I can make x=2y-1, input that into the original equation and that will give me a quadratic to factorise?
10. (Original post by jsk800)
Okay so just 2y-x-1 = 0. I can make x=2y-1, input that into the original equation and that will give me a quadratic to factorise?
Yes.
11. (Original post by ghostwalker)
Yes.
Thank you, most dearestly.
12. (Original post by ghostwalker)
Yes.
Yep, just could not follow his method at all for (c) - laptop crashed so thanks for picking this up
13. checkout the c4 solution bank
14. (Original post by Muttley79)
Yep, just could not follow his method at all for (c) - laptop crashed so thanks for picking this up
For C I was using simultaneous equations to find a common X and y value for both equations, considering that I thought the numerator and denominator were equal to 0. I have learnt how to do it now, thanks to commenter.
15. (Original post by jsk800)
For C I was using simultaneous equations to find a common X and y value for both equations, considering that I thought the numerator and denominator were equal to 0. I have learnt how to do it now, thanks to commenter.
16. (Original post by Muttley79)
No, he said that Dy/DX for example is good because it's mathematically correct for parametric equations when Dy/DX = Dy/dt * dt/DX
17. (Original post by jsk800)
No, he said that Dy/DX for example is good because it's mathematically correct for parametric equations when Dy/DX = Dy/dt * dt/DX
At the beginning it's wrong - you need to say 'differentiating wrt x' it could confuse you in the pressure of the exam having that 'dy/dx' there.

Also I'd be careful how you collect terms - btw, I'm a teacher just trying to help
18. (Original post by Daniel00)
checkout the c4 solution bank
That is foolish - it won't be there in the exam! It's better to struggle and work it through yourself.
19. (Original post by Muttley79)
That is foolish - it won't be there in the exam! It's better to struggle and work it through yourself.
If you cant work it out yourself, dont ask other people thats even more stupid. If you check out the solutionbank you have to learn how it works and understand it, hence gaining greater knowledge rather than getting the answers fed to you. Because they dont tell you why they did something, just shows the working out and you have to figure out why it was done.
20. (Original post by Daniel00)
If you cant work it out yourself, dont ask other people thats even more stupid. If you check out the solutionbank you have to learn how it works and understand it, hence gaining greater knowledge rather than getting the answers fed to you. Because they dont tell you why they did something, just shows the working out and you have to figure out why it was done.
They never tell you the answer here, they will only give a hint or guide me. I've learnt from the replies that to find an infinite gradient (x=constant), I should only equate the denominator to 0, not the numerator. Makes sense, as any number divided by 0 is theoretically infinite.

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