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x = tan (y), get it wrong? watch

1. (Original post by Remarqable M)
that part(question 4) was the easiest of all! lol differentiation is so damn easy! can't wait to move on to integration by recognition
i decided to finally buckle down with C4 integration today, i got so much done - substitution, by parts, partial fractions, differential equations... gonna finish it off tomorrow
2. (Original post by Pheylan)
op will

hey i need an A* ! i think i didnt get much else wrong, so no fail
3. (Original post by steve2005)
What's wrong with the question? It's easy.
It's the fact that one asks for you to find dy/dx using y and x

and the second part, after telling you that x = tan(y), asks for dy/dx again.

So people thought to substitute x from the second part into the first part and then try and find dy/dx! I never perceived it like this at all but looking at it again, I could understand people misinterpreting it in the pressure of the exam.
4. (Original post by steve2005)
What's wrong with the question? It's easy.
It's bad style/notation. In general, if you define x and y by something, you can't then just change what you mean later on. It's obvious that they're doing that here, given the result, but consider this question:

"Given that y = (lnx), find dy/dx. Given that x = 1/k, show that dy/dx = k"

Over here, you would assume that they want you to replace x with 1/k in your previous expression of dy/dx. Likewise, one might assume that they're saying

"dy/dx = whatever. Now sub in x = tany into your previous expression and show that you can use algebra to convert it into dy/dx = 1/(1 + x^2)"

It most definitely is bad form and if I did anything like that in my university maths homework (for abstract maths at least), I'd be raped. What they should have said is "Given that v = tan(u), show that du/dv = whatever", but I suspect they didn't because some students would get confused about the use of u and v rather than x and y.
5. (Original post by .:excel4100%:.)
It's the fact that one asks for you to find dy/dx using y and x

and the second part, after telling you that x = tan(y), asks for dy/dx again.

So people thought to substitute x from the second part into the first part and then try and find dy/dx! I never perceived it like this at all but looking at it again, I could understand people misinterpreting it in the pressure of the exam.
i don't understand lol how can people perceive that the two questions are related
6. (Original post by Swayum)
It's bad style/notation. In general, if you define x and y by something, you can't then just change what you mean later on. It's obvious that they're doing that here, given the result, but consider this question:

"Given that y = (lnx), find dy/dx. Given that x = 1/k, show that dy/dx = k"

Over here, you would assume that they want you to replace x with 1/k in your previous expression of dy/dx. Likewise, one might assume that they're saying

"dy/dx = whatever. Now sub in x = tany into your previous expression and show that you can use algebra to convert it into dy/dx = 1/(1 + x^2)"

It most definitely is bad form and if I did anything like that in my university maths homework (for abstract maths at least), I'd be raped.
ok you've explained it well in the light of this bad notation i expect alot of student to have misintpreted, so the grade boundary will be super loooow
7. (Original post by Remarqable M)
i don't understand lol how can people perceive that the two questions are related
Read Swayum's post above! Under the tension of the exam, anything can happen
8. It was 5 friggin marks, of course I'm gonna think it will be complex.
Add that with all the other nudges in that direction i.e wrong use of letters, i),ii) rather than a) b), "given that" and it becomes quite clear subbing x=tany into the original.
The question before required much more work and it was worth less WTF?!

5 God damn marks for what we finally realise is a stupidly easy question which can be solved by 2 pieces of information and the formula book, how can they justify it? it should have been 2 marks!
9. (Original post by crazedmonkey09)
It was 5 friggin marks, of course I'm gonna think it will be complex.
Add that with all the other nudges in that direction i.e wrong use of letters, i),ii) rather than a) b), "given that" and it becomes quite clear subbing x=tany into the original.
The question before required much more work and it was worth less WTF?!

5 God damn marks for what we finally realise is a stupidly easy question which can be solved by 2 pieces of information and the formula book, how can they justify it? it should have been 2 marks!
don't be sad or frustrated because if you got all your other questions right then there should be no problem best of luck mate! hope you get the grade you deserve
10. (Original post by Swayum)
It's bad style/notation. In general, if you define x and y by something, you can't then just change what you mean later on. It's obvious that they're doing that here, given the result, but consider this question:

"Given that y = (lnx), find dy/dx. Given that x = 1/k, show that dy/dx = k"

Over here, you would assume that they want you to replace x with 1/k in your previous expression of dy/dx. Likewise, one might assume that they're saying

"dy/dx = whatever. Now sub in x = tany into your previous expression and show that you can use algebra to convert it into dy/dx = 1/(1 + x^2)"

It most definitely is bad form and if I did anything like that in my university maths homework (for abstract maths at least), I'd be raped. What they should have said is "Given that v = tan(u), show that du/dv = whatever", but I suspect they didn't because some students would get confused about the use of u and v rather than x and y.
You protest too much.
11. (Original post by steve2005)
You protest too much.
What? I stated something, you asked me to explain myself, so I did and that results in me "protesting" too much? What am I allegedly "protesting" anyway?
12. (Original post by crazedmonkey09)

5 God damn marks for what we finally realise is a stupidly easy question which can be solved by 2 pieces of information and the formula book, how can they justify it? it should have been 2 marks!
Actually if you quoted the derivative of tan^-1x from the formula book you would simply get one mark for realising x = tany -> y = tan^-1x.

You need to show it for the 5 marks.
13. (Original post by Rough Inclined Plane)
hey i need an A* ! i think i didnt get much else wrong, so no fail
I don't think Imperial tend to like whingers.

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