Not sure if mentioned already mentioned but by passing a Triangle wave through a Low Pass filter you are effectively generating an approximation of a Sine wave. You can visualise this with any oscilloscope. The sharp points of the triangle become smoothed the more the cutoff frequency is reduced. In the frequency domain this amounts to the removal of higher order harmonics. There were some similar points made in a thread here some time ago. Even posted some images in there that are relevant
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