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## Orthogonality

The vectors (signals) and are said to be

orthogonalif , denoted . That is to sayNote that if and are real and orthogonal, the cosine of the angle between them is zero. In plane geometry (), the angle between twoperpendicular lines is , and , as expected. More generally, orthogonality corresponds to the fact that two vectors in-space intersect at a

right angleand are thusperpendiculargeometrically.

Example ():Let and , as shown in Fig. 6.8.

The inner product is . This shows that the vectors are

orthogonal. As marked in the figure, the lines intersect at a right angle and are therefore perpendicular.