Question

Which of the given signals are periodic?

(a) x(t) = 4 cos(5πt)

(b) x(t) = u(t) – 1/2

(c) x(t) = 4u(t) + 2sin(3t)

(d) x[n] = 2sin(3n)

I have been asked this question in a job interview.

My doubt stems from Classification of Signals in division Signals and Spectra of Digital Communications

Answer 1

The correct choice is (a) x(t) = 4 cos(5πt)

The explanation: T=2π / 5π = 2/5, periodic with period 2/5.