A question about sampling theory
Given two signals $x_1(t) = \cos(2\pi 2t)$ and $x_2(t) = \cos(2\pi 3t)$ which have frequencies of 2 and 3 Hz respectiveley, if you sample them uniformly with a sampling rate of 5 samples per second (or a sampling period $T$ of $\frac {1}{5}$ = 0.2 seconds) you will get the following two discrete time signals $$x_1[n] = x_1(nT) =  \cos(4\pi \frac {n}{5} )$$ and $$x_2[n] = x_2(nT) = \cos(6\pi \frac {n}{5})$$ 

It can be easily seen with simple trigonometry, ( by using $\cos(x) = \cos(2\pi-x)$) ,  that $x_1[n] = x_2[n]$ which is a manifestation of aliasing that occured on the 3hz signal, therefore, the 3Hz signal seems like 2Hz.