A signal is defined as Assume 4,> 0 for i=1,2,3 and that f2f2fi. (a) State a necessary condition on the frequencies fi. f and f such that x(t) is periodic. (b) State a necessary condition on the amplitudes 4, 42 and A, such that x() is said to be low- pass. x(t) = A sin(2nft) + A sin(2xft) + A3 sin(2n fat) (c) Sketch the spectrum X() of the signal manually. Assuming that the (7-1)th digit of your SID + 3 A = for i=1,2,3. Remark for 03(c): 1" digit and fi = 8th digit the (i + 5) th digit of your SID + 1 0.004 For example, if your SID is 1 2 3 4 5 6 7 8, then A=2.25 -2250