Any periodic signal can be decomposed into Fourier components of sinusoidals at different frequencies. The different frequencies are multiples of the fundamental.
For the triangle wave, tri(f) = constant * (sin(f)-1/9*sin(3f)+1/25*sin(5f)+…). There is a common term of 2 pi t for each sine argument. However for pd that is not relevant. It will be if we are using Python or another language to create the waveforms.
A better approximation will result as we take more terms of the series. It is important to set the phases into the right inlet of osc~.
The mono audio signal shows it is now approximating a triangle wave.