A question about sampling rate of cosine signal
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.