## Bảng chú giải

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# Linear AlgebraVectors

Thời gian đọc: ~20 min

A vector in is a column of real numbers. These real numbers are called the components or entries of the vector.

Example
is a vector in . We say that the first component of is equal to , the second component is equal to , and the third component is equal to .

For typographical convenience, and for consistency with Python Julia we will often write vectors horizontally, like . This notation means the same thing as .

We draw a vector in as an arrow from one point to another so that the horizontal separation between the points is equal to the first component of the vector and the vertical separation between the points is equal to the second component.

We define the norm of a vector to be the length of the associated arrow, which may be calculated as the square root of the of 's components. A vector whose norm is 1 is called a unit vector.

The fundamental vector operations are:

2. Scalar multiplication (multiplication of a real number and a vector).

These operations are defined componentwise, and they have simple geometric interpretations:

1. Summing vectors concatenates them tail-to-head, and

2. Multiplying a vector by a positive real number preserves its direction and multiplies its norm by .

Scalar multiplication is denoted symbolically by placing the scalar adjacent to the vector, and vector addition is denoted with "+" between two vectors. We use the usual notational conveniences from arithmetic, like writing as an abbreviation for .

Exercise
The first component of is equal to and the second component is equal to .

Solution. By definition, we have

Exercise
Determine whether there exists a real number satisfying the vector equation

Solution. For the first component of the two vectors to be equal, the equation would have to hold. This implies that . If we substitute , then the second component on the left-hand side is , so there is no such number.

Exercise
Show that every nonzero vector can be written as the product of a nonnegative real number and a unit vector .

Solution. We can see that the unit vector must point in the same direction as , since multiplying it by does not change its direction. Furthermore, if is the unit vector pointing in the same direction as , then we must scale by a factor of to get . Thus we find that and .

Exercise
Find a formula in terms of and which represents the vector from the head of to the head of when and are situated so that their tails coincide.

Note: Two vectors' tails coincide when they originate from the same point.

Solution. The desired vector has the property that adding it to gives . In other words, , which implies that .

Exercise
Solve for in terms of and in the equation , assuming that and are vectors in and is a nonzero real number.

Solution. We add to both sides and multiply both sides by to get .

Bruno