Discrete signals in spectrum to Analog : DFT
For signal 1 ($x_1(t)$) :
The Fourier's transform ($F(f)$) of $Acos(2\pi ft)$ is $\frac{A}{2}(\delta(f-f_0)+\delta(f+f_0))$ and for $Asin(2\pi ft)$ it's $\frac{jA}{2}(\delta(f-f_0)-\delta(f+f_0))$

Replacing frequencies and amplitudes with the appropriate ones, the Fourier's transform of $x_1(t)$ is :
$$
F(f) = \delta(f-3000 Hz)+\delta(f+3000 Hz)+\frac{j}{2}\left(\delta(f-2000 Hz)-\delta(f+2000 Hz)\right)
$$

Hence the observed Diracs on fig. 1. You did not scale your axis according to the fft sampling while plotting it so the frequency does not appear clearly. The amplitudes are respectively 4 and 8 which is the right ratio so I guess you also did not scale the fft result (matlab doesnt do it itself), it should be 0.5 and 1 according to what I calculated.

For signal 2 it works the same way

edit : there is a mistake in the sampling frequency (1 kHz) you mention. To fit the graphs properly (and respect Shanon) it should be 8 KHz.