| a | * cos θ is called the projection of vector a on vector b; Vector a vector b = | a |||| b | * cos θ (θ is the included angle between the two vectors); | b | * cos θ is called the projection of vector b on vector a.

Tóuyǐng, a mathematical term, refers to the shadow cast by a figure on a surface or a line.

The vector with the same length and opposite direction as A is called the inverse quantity of A, and it is denoted as -a, with -(-A) = A, and the inverse quantity of zero vector is still zero vector.

Non-zero vectors with the same or opposite directions are called parallel lines (or * * * lines) vectors. Vectors A and B are parallel (* * * line), which is represented as A ∨ B. The length of zero vector is zero, and it is a vector with the same starting point and ending point, and its direction is uncertain. We stipulate that the zero vector is parallel to any vector. A set of vectors parallel to the same line is a * * * line vector.

If a=(x, y) and b=(m, n), then a//b→a×b=xn-ym=0.