1. The problem statement, all variables and given/known data r(t)=(cos t +tsint)i + (sin t -tcost)j + 3k 2. Relevant equations dr/dt=v(t) 3. The attempt at a solution when I take the derivative of r(t) I get; v(t)=(-sin t + tcost)i +(cos t +tsint)j the book says; v(t)=(t cos t)i +(tsint)j could some one tell me why? Where did the -sint and the cos t go? I guess it is an identity but I don't see it.