What is the variance of the difference between two successive signal s

1.	What is the variance of the difference between two successive signal samples, d(n) = x(n) ax(n-1)?
A.	( _d^2=2 _x^2 [1-a^2] )
B.	( _d^2= _x^2 [1+a^2] )
C.	( _d^2= _x^2 [1-a^2] )
D.	( _d^2=2 _x^2 [1+a^2] )
Answer» D. ( _d^2=2 _x^2 [1+a^2] )

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