Theorem 12.8 Let A be an n × n matrix. The following statements are equivalent. 1. The columns of the matrix A form an orthonormal set. 2. A−1 = At . 3. For vectors x and y, ⟨Ax, Ay⟩ = ⟨x, y⟩. 4. For vectors x and y, ∥Ax − Ay∥ = ∥x − y∥. 5. For any vector x, ∥Ax∥ = ∥x∥.

Algebra & Trigonometry with Analytic Geometry
13th Edition
ISBN:9781133382119
Author:Swokowski
Publisher:Swokowski
Chapter9: Systems Of Equations And Inequalities
Section9.6: The Algebra Of Matrices
Problem 14E
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Theorem 12.8 Let A be an n × n matrix. The following statements are equivalent.
1. The columns of the matrix A form an orthonormal set.
2. A−1 = At
.
3. For vectors x and y, ⟨Ax, Ay⟩ = ⟨x, y⟩.
4. For vectors x and y, ∥Ax − Ay∥ = ∥x − y∥.
5. For any vector x, ∥Ax∥ = ∥x∥.

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