Integral is area under the curve.
During rising slope of triangular wave, the following 2 points may be noted.
1. The area under the curve keeps increasing.
2. As the wave is triangular, the rate at which the area increases, also increases. As captured in the image, this can be seen by seeing the slope of the integral (upper waveform). It may be observed that the slope of the integral is continuously increasing.
During falling slope of triangular wave, following 2 points may be noted.
1. As the value of wave is above zero, the area (i.e. the integral) keeps increasing. From the image, this may be inferred by seeing that the integral waveform is always increasing.
2. Unlike in the rising case, the rate at which the area under the curve increases, actually decreases. It may be observed that the slope of the integral in this region is continuously increasing.