Determinant of a quadratic form.

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Is there any visual or not, interpretation of the determinant of a quadratic form? I think it helps me to understand why the determinant is not an invariant under a change of basis. Because the determinant of a matrix of a linear map means scaling the area of a parallelogram, so it doesn't matter which basis we chose.

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The fact that only the square class of the determinant is well-defined should tell you that it is not really a geometric, but rather an algebraic invariant. For instance if you work over the real numbers only its sign is well-defined.