Question 2 Let V be an inner product space with inner norm ||| = VI·I. Prove the parallelogram law ||I+ y||² + ||x – y|l = 2 (||7||² + ||y|²) - This can be interpreted as saying that the sum of the squares of the lengths of the four sides of a parallelogram equals the sum of the squares of the di- agonals.

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### The Parallelogram Law in Inner Product Spaces

**Question 2**

Let \( V \) be an inner product space with an inner norm \(\| x \| = \sqrt{x \cdot x}\). Prove the parallelogram law.

The parallelogram law states:

\[
\| x + y \|^2 + \| x - y \|^2 = 2 \left( \| x \|^2 + \| y \|^2 \right).
\]

This can be interpreted as saying that the sum of the squares of the lengths of the four sides of a parallelogram equals the sum of the squares of the diagonals.

### Explanation

In the above formula:
- \(\| x \|\) represents the norm or the length/magnitude of vector \( x \) in the inner product space.
- \(\| x + y \|\) denotes the norm of the vector resulting from the addition of vectors \( x \) and \( y \).
- \(\| x - y \|\) denotes the norm of the vector resulting from the subtraction of vector \( y \) from vector \( x \).

The parallelogram law demonstrates a fundamental property of inner product spaces, relating to how lengths (or norms) behave under addition and subtraction of vectors. This law is foundational in linear algebra and vector space theory.

### Intuition Behind the Parallelogram Law

Geometrically, imagine a parallelogram:
- The vectors \( x \) and \( y \) can be thought of as the sides of the parallelogram.
- The vectors \( x + y \) and \( x - y \) represent the diagonals of the parallelogram.

The parallelogram law then tells us that the combined length of the diagonals, measured as the sum of the squares of their lengths, is equal to twice the sum of the squares of the lengths of the sides. This corresponds to:
- Summing the distances along the geometry's diagonals.
- Reflecting the inherent symmetry and properties preserved in the structure of inner product spaces.
Transcribed Image Text:### The Parallelogram Law in Inner Product Spaces **Question 2** Let \( V \) be an inner product space with an inner norm \(\| x \| = \sqrt{x \cdot x}\). Prove the parallelogram law. The parallelogram law states: \[ \| x + y \|^2 + \| x - y \|^2 = 2 \left( \| x \|^2 + \| y \|^2 \right). \] This can be interpreted as saying that the sum of the squares of the lengths of the four sides of a parallelogram equals the sum of the squares of the diagonals. ### Explanation In the above formula: - \(\| x \|\) represents the norm or the length/magnitude of vector \( x \) in the inner product space. - \(\| x + y \|\) denotes the norm of the vector resulting from the addition of vectors \( x \) and \( y \). - \(\| x - y \|\) denotes the norm of the vector resulting from the subtraction of vector \( y \) from vector \( x \). The parallelogram law demonstrates a fundamental property of inner product spaces, relating to how lengths (or norms) behave under addition and subtraction of vectors. This law is foundational in linear algebra and vector space theory. ### Intuition Behind the Parallelogram Law Geometrically, imagine a parallelogram: - The vectors \( x \) and \( y \) can be thought of as the sides of the parallelogram. - The vectors \( x + y \) and \( x - y \) represent the diagonals of the parallelogram. The parallelogram law then tells us that the combined length of the diagonals, measured as the sum of the squares of their lengths, is equal to twice the sum of the squares of the lengths of the sides. This corresponds to: - Summing the distances along the geometry's diagonals. - Reflecting the inherent symmetry and properties preserved in the structure of inner product spaces.
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