Question

PLEASE HELP!!!!!!!
Find the vectors T, N, and B at the given pointr(t) =
<7cost, 7sint, 7lncost>
(7, 0, 0)

Answer 1

it's the vector in 3D coordinate.

Answer 2

yes

Answer 3

I think I can find the unit tangent vector I just don't know how to use this to find a value for t

Answer 4

hey, 7 cost = 7 mean cost =1
7 sin t =0 means sint =0
which angle give you that value? 0, or 180, right?

Answer 5

Yes.... I was trying to do all kinds of fancy substitution and inverse trig identities..... Please tell me it is not that simple

Answer 6

I am sorry to say, it's that simple

Answer 7

so then t is not a numeric value?? it is actually a degree or radian?

Answer 8

to me, it's a vector along the positive x axis with magnitude 7

Answer 9

I just ask because when I solve for the unit tangent vector....."T"....I will have the derivative of the vector valued function over a magnitude of the derivative of the vector valued function. Then to find the actual value I will have to plug in a value for t. Then again when I find the unit normal vector I will need to plug in a value for t. So after you said that I was thinking if the derivatives are all trig functions then I can plug in a degree or radian but what happens if It doesn't give me all trig functions/ In this case I think it will, but what if it does not, or what if that is not the case on another problem?

Answer 10

so, just post your original problem in Math, many people good at Math will help you. Cal 3 is not hard to them. Remember, original problem, not just the part you don't understand. Because sometimes, we make mistake unconsciously which leads to unsolvable problem.

Answer 11

wow I did not even realize I was in Physics, but thank you for our help.
By the way that was the full original problem copied and pasted from my assignment, It even had the PLEASE HELP!!!!! :)

Answer 12

you have r(t) = ( 7cos t, 7 sin t , 7 ln cos t)
and formula T = \( \dfrac{r'(t)}{|r(t)|}\) just apply , take derivative part by part.
r'(t) = (- 7sint, 7 cost , -tant) and |r(t)|= \(\sqrt{(7cost)^2 + (7sint)^2 + (7ln cost)^2}\) that's it.

Answer 13

@Loser66 just one other question.... when you have a vector valued function and a point you are allowed to set the i th component equal to the x-value of the point?

Answer 14

yes

Answer 15

@Loser66 can you look at another part of this problem? I posted it in the math section but no one over there is touching it. http://openstudy.com/updates/523b8535e4b0fbf3cc7b4fba