Power of (−1)^n u[n]

Correct Answer

B — 0.5

Concept and Reasoning

For n≥0, |(−1)^n|²=1; for n<0 the unit step makes the signal zero. Using the standard symmetric average from −N to N gives P=lim(N+1)/(2N+1)=1/2=0.5. Thus option B is correct.

Calculation / Key Relation

P=lim[N→∞](1/(2N+1))Σ|x[n]|²=1/2

Exam Takeaway

The unit step makes only about half of the symmetric time axis nonzero.

Common Mistake

Do not answer 1 merely because the nonzero samples have unit magnitude.