A question about sampling rate of cosine signal Given $$c(t) = \cos(2\pi\cdot 30 \cdot t) $$

If we sample this signal at the Nyquist rate 60 Hz and at a higher rate of 80 Hz, we get the following:



There is no aliasing as $f$ = 30 Hz is less than or equal to the folding frequency, 30 Hz and 40 Hz, respectively.

Why does sampling at a rate of 80 Hz result in this "distorted" cosine signal, although it is higher than the Nyquist rate? (I know if we go way higher than 80 Hz the signal will look better.)
It is actually not distorted, it is sampled at high enough rate. What fools you is the straight lines drawn between sample points, it gives you a false impression of the waveform. It shows you a linear interpolation of the signal. It does not represent how the signal would actually look like. A sampled signal exists only at the sample points, and to convert that into continuous analog representation that also exists between the sample points, you should use sinc interpolation which limits the bandwidth to half of the sampling rate.