The directional derivative of at the point along the direction is the scalar (inner) product of and .............................  ( is the unit vector with the same direction as )

Let and . The function decreases most rapidly in the direction of .......

The function     in the direction of gradient vector.

Let is a stacionary point of a twice differentiable function , and are the principal minors of .

The tangent line of a curve at the point is

Let . The matrix of the derivative of   at the point (i.e.  ) has size

If is a scalar field, then is called

Let . The matrix of the derivative of   at the point (i.e.  ) has size

The directional derivative of at the point along the direction is the  of   and . ( is the unit vector with the same direction as )