4. (a) S1(.75)=-1, S1(1.5)=0, S1(2.25)=1, S1(3)=0, S1(3.75)=-1, S1(4.5)=0
(b) At all times indicated, S2(t)=0.
(c) From (b) we see at the times indicated, S3(t)=S1(t).
(d) The plot below shows S3(t) repeats after t increases by 3, so the frequency of S3(t) is 1/3 Hz. Comparing this with (c), we see some care must be exercised in making inferences: unluckily chosen sampling times can give misleading results.

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