Chapter 3: Problem 26
Prove that the vectors \(\|v\| w+\|w\| v\) and \(\|v\| w-\|w\| v\) in an inner- product space \(V\) are perpendicular.
Short Answer
Expert verified
Vectors \(\|v\| w+\|w\| v\) and \(\|v\| w-\|w\| v\) are perpendicular because their inner product is zero.
Step by step solution
01
Understand Vector Notation and Definitions
The vectors given are \(\|v\| w+\|w\| v\) and \(\|v\| w-\|w\| v\), where \(v\) and \(w\) are vectors in an inner-product space \(V\). The notation \(\|v\|\) represents the norm of the vector, which is defined as \(\sqrt{\langle v, v \rangle}\), where \(\langle v, v \rangle\) is the inner product of \(v\) with itself.
02
Define Perpendicular Vectors in Inner-product Spaces
Two vectors \(a\) and \(b\) in an inner-product space \(V\) are said to be perpendicular (or orthogonal) if their inner product equals zero: \(\langle a, b \rangle = 0\). We need to show that \(\langle \|v\| w+\|w\| v, \|v\| w-\|w\| v \rangle = 0\).
03
Calculate the Inner Product
Compute the inner product of the two vectors:\[langle \|v\| w+\|w\| v, \|v\| w-\|w\| v \rangle = \langle \|v\| w, \|v\| w \rangle - \langle \|v\| w, \|w\| v \rangle + \langle \|w\| v, \|v\| w \rangle - \langle \|w\| v, \|w\| v \rangle\]Due to properties of inner products, the second and third terms simplify to 0.
04
Simplify Using Inner-product Properties
Apply the linearity and symmetry properties of the inner product:1. \(\langle \|v\| w, \|v\| w \rangle = \|v\|^2 \langle w, w \rangle \)2. \(\langle \|v\| w, \|w\| v \rangle = \|v\|\|w\| \langle w, v \rangle \)3. \(\langle \|w\| v, \|v\| w \rangle = \|v\|\|w\| \langle v, w \rangle \ = \langle \|v\| w, \|w\| v \rangle\)4. \(\langle \|w\| v, \|w\| v \rangle = \|w\|^2 \langle v, v \rangle \)Noticing the symmetry, the negative terms cancel with the positive terms.
05
Confirm Orthogonality
Given the simplifications:\[\langle \|v\| w+\|w\| v, \|v\| w-\|w\| v \rangle = \|v\|^2 \langle w, w \rangle - \|w\|^2 \langle v, v \rangle = 0\]This means the vectors are orthogonal, thus confirming \(\|v\| w+\|w\| v\) and \(\|v\| w-\|w\| v\) are perpendicular.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Perpendicular Vectors
Perpendicular vectors, also known as orthogonal vectors, play a pivotal role in inner-product spaces. When two vectors are perpendicular, their inner product is zero. This concept, \(\langle a, b \rangle = 0\), is crucial for understanding many areas of linear algebra and geometry.
- Geometric Interpretation: Imagine two vectors as arrows meeting at a right angle; they form an 'L' shape. Perpendicular vectors extend this idea into multiple dimensions.
- Application: In physical space, perpendicular vectors are common in force decompositions and changes of base.
Vector Norms
The norm of a vector is a measure of its length or magnitude in an inner-product space. It gives a sense of how big a vector is. For any vector \(v\), the norm is represented as \(|v| = \sqrt{\langle v, v \rangle}\). This formula derives from the inner product of a vector with itself.
- Intuition: Consider the norm as the "size" of the vector. If you imagine \(v\) as a directed line segment from the origin, \(|v|\) is its geometrical length.
- Properties: A vector's norm is always a non-negative scalar. Importantly, if \(|v| = 0\), the vector is the zero vector, which means it has no direction or length.
- Applications: Vector norms are widely used in scaling vectors to unit length, which is crucial for normalization processes in mathematics and data science.
Orthogonality
Orthogonality is a broad concept stemming from perpendicularity that applies to vectors more generally. Two vectors in an inner-product space are orthogonal when their inner product is zero. This concept extends beyond simple right-angle relationships.
- Importance: Orthogonal vectors often represent independent directions in vector space, making them fundamental in algorithms like Gram-Schmidt and techniques like singular value decomposition.
- Orthogonal Sets: A set of vectors is orthogonal if each pair of vectors in the set is orthogonal. This means no vector in the set can be written as a linear combination of others, a useful property for constructing orthonormal bases.
- Practical Use: Orthogonal vectors simplify many calculations, showing their importance in simplifying linear transformations and solving equations.
Inner Product Properties
Inner-product spaces are defined by their inner product, which has specific properties. Understanding these properties can simplify proving vector relationships such as perpendicularity, among others.
- Linearity: The inner product is linear in its first and second arguments separately. This means functions like \(\langle au + bv, w \rangle = a \langle u, w \rangle + b \langle v, w \rangle\).
- Symmetry: For any vectors \(v\) and \(w\), \(\langle v, w \rangle = \langle w, v \rangle\). This property is particularly meaningful in simplifying expressions where swapping terms might reduce complexity.
- Positive Definite: The inner product of a vector with itself is always non-negative, \(\langle v, v \rangle \geq 0\), and \(\langle v, v \rangle = 0\) if and only if \(v\) is the zero vector.