# Formulas and Tables

Analytic Geometry# Dot Product

Vector lengths: \(\left| \mathbf{u} \right|,\) \(\left| \mathbf{v} \right|\)

Null vector: \(\mathbf{0}\)

Unit vectors: \(\mathbf{i},\) \(\mathbf{j},\) \(\mathbf{k}\)

Coordinates of vectors: \({X_1},\) \({Y_1},\) \({Z_1},\) \({X_2},\) \({Y_2},\) \({Z_2}\)

Real numbers: \(\lambda,\) \(\mu\)

- The dot product (also called the scalar product) of two vectors \(\mathbf{u}\) and \(\mathbf{v}\) is the product of their lengths and the cosine of the angle between them.

\(\mathbf{u} \cdot \mathbf{v} = \left| \mathbf{u} \right|\left| \mathbf{v} \right|\cos \theta \)

- Dot product in coordinate form

If \(\mathbf{u} = \left( {{X_1},{Y_1},{Z_1}} \right)\), \(\mathbf{v} = \left( {{X_2},{Y_2},{Z_2}} \right),\) then

\(\mathbf{u} \cdot \mathbf{v} = {X_1}{X_2} \) \(+\; {Y_1}{Y_2} \) \(+\; {Z_1}{Z_2}\). - Angle between two vectors

If \(\mathbf{u} = \left( {{X_1},{Y_1},{Z_1}} \right)\), \(\mathbf{v} = \left( {{X_2},{Y_2},{Z_2}} \right)\), then

\(\cos \theta = {\large\frac{{\mathbf{u} \cdot \mathbf{v}}}{{\left| \mathbf{u} \right| \cdot \left| \mathbf{v} \right|}}\normalsize} =\) \( {\large\frac{{{X_1}{X_2} + {Y_1}{Y_2} + {Z_1}{Z_2}}}{{\sqrt {X_1^2 + Y_1^2 + Z_1^2} \sqrt {X_2^2 + Y_2^2 + Z_2^2} }}\normalsize}.\)

It is assumed here that \(\mathbf{u}\) and \(\mathbf{v}\) are non-zero vectors. - Commutative law for dot product

\(\mathbf{u} \cdot \mathbf{u} = \mathbf{v} \cdot \mathbf{u}\) - Associative law for dot product

\(\left( {\lambda \mathbf{u}} \right) \cdot \left( {\mu \mathbf{v}} \right) =\) \( \lambda \mu \mathbf{u} \cdot \mathbf{v}\) - Distributive law for dot product

\(\mathbf{u} \cdot \left( {\mathbf{v} + \mathbf{w}} \right) =\) \( \mathbf{u} \cdot \mathbf{v} \) \(+\; \mathbf{u} \cdot \mathbf{w}\) - The dot product of the vectors \(\mathbf{u}\) and \(\mathbf{v}\) is zero if the vectors \(\mathbf{u}\) and \(\mathbf{v}\) are perpendicular, or if one of the vectors \(\mathbf{u}\) and \(\mathbf{v}\) or both of them are zero.

\(\mathbf{u} \cdot \mathbf{v} = 0,\) if \(\mathbf{u} \bot \mathbf{v}\left( {\theta = \large\frac{\pi }{2}}\normalsize \right)\) or \(\mathbf{u} = \mathbf{0}\) and/or \(\mathbf{v} = \mathbf{0}\). - The dot product of two vectors \(\mathbf{u}\) and \(\mathbf{v}\) is positive if the angle \(\theta\) between the vectors \(\mathbf{u}\) and \(\mathbf{v}\) is acute.

\(\mathbf{u} \cdot \mathbf{v} \gt 0\), if \(0 \lt \theta \lt {\large\frac{\pi }{2}\normalsize}\). - The dot product of two vectors \(\mathbf{u}\) and \(\mathbf{v}\) is negative if the angle \(\theta\) between the vectors \(\mathbf{u}\) and \(\mathbf{v}\) is obtuse.

\(\mathbf{u} \cdot \mathbf{v} \lt 0\), if \({\large\frac{\pi }{2}\normalsize} \lt \theta \lt \pi\). - The dot product of two vectors is less than or equal to the product of their lengths:

\(\mathbf{u} \cdot \mathbf{v} \le \left| \mathbf{u} \right| \cdot \left| \mathbf{v} \right|\) - The dot product of two vectors \(\mathbf{u}\) and \(\mathbf{v}\) is equal to the product of their lengths if only the vectors \(\mathbf{u}\) and \(\mathbf{v}\) are parallel:

\(\mathbf{u} \cdot \mathbf{v} = \left| \mathbf{u} \right| \cdot \left| \mathbf{v} \right|,\) if \(\mathbf{u}\parallel \mathbf{v}\left( {\theta = 0} \right)\). - The scalar square of a vector is equal to the square of its length:

If \(\mathbf{u} = \left( {{X_1},{Y_1},{Z_1}} \right),\) then \(\mathbf{u} \cdot \mathbf{u} = {{\mathbf{u}}^2} =\) \( {\left| \mathbf{u} \right|^2} =\) \( X_1^2 + Y_1^2 + Z_1^2.\) - Scalar squares of the unit vectors

\(\mathbf{i} \cdot \mathbf{i} = \mathbf{j} \cdot \mathbf{j} =\) \( \mathbf{k} \cdot \mathbf{k} \) \(= 1\) - Dot product of distinct unit vectors

\(\mathbf{i} \cdot \mathbf{j} = \mathbf{j} \cdot \mathbf{k} =\) \( \mathbf{k} \cdot \mathbf{i} \) \(= 0\)