Derivation of parametric function

1. Jan 18, 2007

kasse

I get the first derivative correct, but what's wrong with my attempt to find the second derivative?

2. Jan 18, 2007

Schrodinger's Dog

a) the bottom line is wrong

should be $$\frac {cos(t)-1}{(1-cos(t))^2}$$

and b) you need to further simplify to get the answer.

EDIT:

What is $$1-cos^2 (39)$$ what is $$(1-cos(39))^2$$

Thus.

$$(1-cos(t))(1+cos(t))\not= 1-cos{^2}(t)$$

Last edited: Jan 19, 2007
3. Jan 18, 2007

cristo

Staff Emeritus
You have a mistake in the denominator: $$(1-cost)^2 \not=1-cos^2t$$

4. Jan 18, 2007

kasse

The answer is supposed to be -1/a(1-(cos(t))^2)
Whatever I do, I don't get an answer including a.

5. Jan 18, 2007

kasse

OK, but this is the right answer, just that I have to simplify?

6. Jan 18, 2007

Schrodinger's Dog

Yep, expand the denominator, and then simplify from there. Your answer is correct apart from the obvious mistake, but it's often better to present the most simple form.

If you can get this I'd be surprised

$$\frac{cos(t)-1}{(1-cos(t))^2}$$ correct.

$$\frac {1}{(cos(t)-1)}$$

Can you work out how you would get this simplification from expanding the denominator in your first answer? -1 as the numerator, is correct but it can be made even simpler.

Last edited: Jan 18, 2007
7. Jan 18, 2007

kasse

Well, my book says:

second derivative = ((cos t)(1-cos t) - (sin t)(sin t))
/ ((1-cos t)^2*a(1-cos t))

= -1/(a(1-cos t)^2)

Heres the page http://www.badongo.com/pic/422090 (example 6)

Mystic...

Last edited: Jan 18, 2007
8. Jan 19, 2007

Schrodinger's Dog

OK I'll check my result with someone else, but I'm pretty sure on this one. a is a constant and since 3 sin(29)/3 cos(29) is the same as saying sin/cos I don't see why it belongs in the second order derivative.

$$\frac {cos(t)-1}{(1-cos(t))^2}$$

What do you end up with?

I checked it on a maths web site and:-

$$\frac{d}{dx} \frac {sin (t)}{1-cos(t)} = \frac {1}{(cos(t)-1)}dx$$

Last edited: Jan 19, 2007
9. Jan 19, 2007

cristo

Staff Emeritus
Schrodinger's Dog is correct. There appears to be a factor of y in the denominator of the middle term. This is a typo.

10. Jan 19, 2007

Schrodinger's Dog

Just goes to show everyones human, even the editors and authors of text books.

I also asked someone else where I work, since I work in a med physics dept, there's usally someone around who still remembers doing calculus without breaking into a cold sweat and they said I was right.

11. Jan 19, 2007

kasse

Cool, I beat the textbook!

12. Jan 19, 2007

Schrodinger's Dog

Well kind of, but you made a typo of your own, you just needed to simplify after expanding the denominator
(1-cos(t))^2 which is (1-cos(t))(1+cos(t)), of course you can check this by multiplying.

$$\frac {cos(t)-1}{(1-cos(t))^2} => \frac {cos(t)-1}{(1-cos(t))(1+cos(t))} => \frac {-1}{1-cos(t)} => \frac {1}{cos(t)-1}$$

Last edited: Jan 19, 2007
13. Jan 20, 2007

kasse

x=(cos t)^3
y=(sin t)^3

dy/dx = -tan t
(dy/dx)' = -1/(cos t)^2

Right?

I can simply derivate the dy/dx function even if it's a function of t and not x?

14. Jan 20, 2007

kasse

I think the book is right after all. Formula nr. (5) is correct, and that's why you also have to multiply with a(1-cos t) in the denominator, since dx/dt = a(1-cos t)

15. Jan 20, 2007

cristo

Staff Emeritus
Yes, sorry, I wasn't really looking at this properly the first time, and just glanced at Schrodinger's Dog's attempt and it looked right; however it is not. What he was doing was taking $$\frac{d^2y/dt^2}{d^2x/dt^2}$$, however this is of course not correct, since this does not equal d2y/dx2. You should follow your textbook. The correct way to approach the second derivative is to use the chain rule; letting y'=dy/dx: $$\frac{dy'}{dt}=\frac{dy'}{dx}\frac{dx}{dt}$$ which then gives the formula for the second derivative of y wrt x:$$\frac{d^2y}{dx^2}=\frac{dy'/dt}{dx/dt}$$.

16. Jan 20, 2007

cristo

Staff Emeritus
No, this is not correct. The red part above is dy'/dt. Plug this into the formula for the second derivative with respect to x.

17. Jan 22, 2007

Schrodinger's Dog

That's my fault I could not read his text book or the question properly, I think I need an eye test, apologies.

also I simply typed the equation in the first part in and got the first and second derivative without a. Which is my answer, but not it seems what the question is asking.

In other words since the first derivative simplified is

$$\frac {a(sin-(t))}{a(1-cos(t))} = \frac{sin-(t)}{1-cos(t)}$$

I simply did a derivation of the second part instead of the first.

In other words my answer is the answer you get if you ignore the constant, which of course is not what the question was asking but then I misread it completely anyway. Like I say eye test time

in real life if I had to derive this particular one.

$$\frac{d^2y}{dx^2} \frac{a(cos(x))}{a(sin(x))}$$ as it's the same as saying $$\frac{a(2)}{a(4)} = \frac{2}{4}$$ if you see what I mean.

I'd ignore the constant a. But this aint real life and that's not what the question wants you to do. Oops apologies.

Last edited: Jan 22, 2007