This isn't clear for me :(
They're showing you the same wave at 2 different times.
I THINK the top wave (the one that starts on the left and goes upwards first) is half a cycle out of phase with the bottom wave.(starts on left and goes down first) If we remove the bottom wave completely, and slide the whole wave left until the point that the top wave hits the middle line on the way down is on the left side, then now the wave looks just like the bottom wave. The top and bottom wave are both the same wave, just drawn out of phase and together.
Yeah,I got it now
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