Homework Help: Find dy/dx as a function of t for the parametric equations

1. Sep 5, 2013

PsychonautQQ

1. The problem statement, all variables and given/known data
find dy/dx as a function of t for the parametric equations
x=cos^7(t)
y=6sin^2(t)

2. Relevant equations

3. The attempt at a solution
well i'm looking for dy/dx.. so first i found dy
dy = 12sin(t)cos(t)

and dx
dx = -7cos^6(t)sin(t)

dy/dx = 12sin(t)cos(t) / -7cos^6(t)sin(t)
(-12/7cos^5(t))

And then it asks for if the curve is concave up or concave down (besides where it is undefined). When i graph it on wolfram alpha it gives both up and down concaves so it leads me to believe I did this wrong. Help

2. Sep 5, 2013

CompuChip

dy/dx is the derivative of y with respect to x, it is not technically a fraction. In many cases you can treat it as such, but this is one of those cases where it doesn't really work.

Does the chain rule ring a bell when written as
$$\frac{dy}{dx} \frac{dx}{dt} = \frac{dy}{dt}$$
or, more conveniently for you,
$$\frac{dy}{dx} = \frac{ \frac{dy}{dt} }{ \frac{dx}{dt} } \text{ ?}$$

3. Sep 5, 2013

eumyang

To find the concavity you need the second derivative. I only see the first derivative in your post. Also, when you say that you found "dy" and "dx", you should say instead "dy/dt" and "dx/dt".

EDIT: Regarding my second point, CompuChip beat me to it. :tongue:

4. Sep 5, 2013

PsychonautQQ

It does not. I just started calculas 3 and we are going over alot of calculus one stuff which i got a D in over 5 years ago. lol ;-(. Calc 2 i stepped up my game and got an A but alot of this stuff is confusing me. I'll go relearn the chain rule. That dt is some kind of new variable that comes out of using the chain rule?

5. Sep 5, 2013

PsychonautQQ

dy/dx = (dy/dt) / (dx/dt). I don't understand how this will get me a result different from what I did the first time. Isn't dy/dt 12sin(t)cos(t)? and dx/dt is 7cos^6(t)sin(t)?

6. Sep 5, 2013

eumyang

I didn't say that your first derivative was wrong (although you forgot a negative in dx/dt in your more recent post). I'm saying that to test for concavity you need to find the second derivative.

7. Sep 5, 2013

MrAnchovy

It won't - in this problem. But if you start writing things like dy = 12sin(t)cos(t)
1. it looks like you don't know what you are doing
2. when problems get harder (like in the second half of this question) you will get caught out

As already stated you need to look at $\frac{d^2y}{dx^2}$ to get the curve's concavity.

8. Sep 5, 2013

PsychonautQQ

okay.
so dy/dx = -12/(7cos^5(t))
I need to take the derivative again to find the concavity
(d/dt)(dy/dx) = (d/dt)(-12/(7cos^5(t)) = (-60/7)tan(t)sec^5(t). And yet when i graph this curve, it looks like it has sections where it concaves up and some where in concaves down.

9. Sep 5, 2013

PsychonautQQ

alright so i see my above post is wrong.. Now i have
(d/dt)(dy/dx) / (dx/dt) = the second derivative i'm looking for.. after doing all the math i got (-60/49)sec^12(t).. anyone have advice?

10. Sep 5, 2013

PsychonautQQ

I GOT IT thanks everyone :D