What is the energy density spectrum of the signal x(n)=anu(n), |a|<1?

1.	What is the energy density spectrum of the signal x(n)=anu(n), \|a\|<1?
A.	( frac{1}{1+2acos +a^2} )
B.	( frac{1}{1-2acos +a^2} )
C.	( frac{1}{1-2acos -a^2} )
D.	( frac{1}{1+2acos -a^2} )
Answer» C. ( frac{1}{1-2acos -a^2} )

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