So we are given T(t) = c'(t)/||c'(t)|| as well as ||T|| = 1 We also know T(t)dotT(t) = 1 and T'(t)dotT(t) = 0 The problem asks us to find T'(t) I tried differentiating c'(t)/||c'(t)|| treating ||c'(t)|| as the square root of the dot product of c'(t) with itself. I used the product rule, chain rule, quotient rule, and ended up with some nasty terms, namely c'(t) dot c"(t). I am pretty sure the answer we are looking for is T'(t) = c"(t). Therefore, if we can prove that T(t) = c'(t), then the answer T'(t) = c"(t) follows. Please help! LOL Not being able to solve this has been bothering me big time!