What is the RMS value of a periodic signal? When a periodic signal is generated by a source connected to a load, a resistor for example, the RMS value is the continuous signal, the DC value which would deliver the same power to the load as the periodic signal.
This article shows how to derive the RMS value of triangle waveforms with different shapes and duty cycles.
The triangle waveform in Figure 1 has a slower rise time than the fall time. In this case, the fall time is small so that it can be considered zero. If it is not zero, read further on deriving the RMS value of a triangle with comparable rise and fall times.