I'm asked to sample the signal $$x_a(t) = \cos{(2\pi300t)} + \cos(2\pi600t)$$ with sampling frequency $F_s = 1000$ and plot the magnitude spectrum for the resulting sampled signal.
My thinking is that the frequency of $300$ does not change, so it just results in the normalized frequency $\frac{3}{10} = 0.3$.
Since $600$ is above the nyquist frequency, aliasing arises. So we get the frequencies $\frac{6}{10} = 0.6$ and $\frac{6}{10} - 1 = - 0.4.$
So in total I would plot peaks at the frequencies $\pm0.3, \pm0.4, \pm0.6$, but the answer is supposedly:
Why? Where is my thinking wrong? And what is the method to always get the right aliasing peaks?