Question 1165603
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This transformation leaves UNCHEANGABLE the vectors  (x,y)  in the plane with x=y

that lie on the line  y=x.  

Hence, these vectors  (x,x)  are eigenvectors with the eigenvalue of 1.



This transformation CHANGEs the vectors  (x,y)  in the plane with x=-y

that lie on the line  y=-x,  orthogonal to the line  x=y.  

The transformation transforms each such a vector into the opposite one.

Hence, these vectors  (x,-x) are eigenvectors with the eigenvalue of -1.
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Solved, answered and explained.