ABSTRACT
For an arbitrary curve y and for any point P E y, similarly to the definitions given in chapter 4 one can define two rays tangent to y at P and the straight line tangent to y at P. If y is smooth, then for any point P there exists a unique straight line tangent to y at P. Modifying slightly the proof of the corresponding statement from chapter 4, one can demonstrate that if r = r(t) is the position vector of y, then rl(t) is the directing vector of the tangent straight line.
