# Thread: find T(t), N(t), at the given time t for the plane curve

1. ## find T(t), N(t), at the given time t for the plane curve

find T(t), N(t), $a_t , a_n$ at the given time t for the plane curve r(t)

$r(t)= ti + \frac{1}{t} j, t = 1$

The book gives this formula for T(t)

$T(t) = \frac {r'(t)}{ || r'(t) ||}$

So, i find $r' = i - \frac {1}{t^2} j$

then the magnitude
$||r'(t)|| = \sqrt{ (\frac{1^2 + 1^2}{t^2})^2} = \sqrt{ \frac{2}{t^4}}$

then, combining for T(t) the solutions manual has this which i cant seem to understand.

$T(t) = \frac {t^2} {\sqrt{ t^4 + 1}} ( i - \frac{1} {\sqrt{ {t^2}}}j) = \frac{1} {\sqrt{ t^4 + 1}}(t^2i - j)$

I am completely lost!!!

2. ## Re: find T(t), N(t), at the given time t for the plane curve

Originally Posted by icelated
find T(t), N(t), $a_t , a_n$ at the given time t for the plane curve r(t)

$||r'(t)|| = \sqrt{ (\frac{1^2 + 1^2}{t^2})^2} = \sqrt{ \frac{2}{t^4}}$
this is wrong,

$||r'(t)||=\sqrt{1^2+\left(\frac{1}{t^2}\right)^2}= \frac{\sqrt{t^4+1}}{t^2}$ as in $r=ai+bj$ and $|r|=\sqrt{a^2+b^2}$