Question

how to find unit tangent vector for (cos t,sin t,sin t) ?

Answer 1

ok.after that how to find the principal unit normal vector ?

Answer 2

doesn't it matter in what what direction?
Del(cost,sint,sint)=(-sint i, cost j, cost k)
magintude of Del(f(x)):Del(f(x))/sqrt[(-sint)^2+(cost)^2+(cost)^2]
(-sint i, cost j, cost k)/sqrt[(cost)^2+1]
??????????????????????
kind of a guess...

Answer 3

I gave you T(t) = f'(t)/IIf(t)II(unit tangent vector)
The principle unit normal is T'(t)/IIT(t)II
where IIf(t)II=magintude of f(t)
http://www.ltcconline.net/greenl/courses/202/vectorFunctions/tannorm.htm