A sine wave with zero amplitude after a 3/4 cycle and increasing is called a "rectified sine wave." It is a special type of waveform that has the shape of a sine wave for the first three-quarters of a cycle and then remains flat (zero amplitude) for the remaining one-quarter of the cycle before increasing again.
The mathematical expression for a rectified sine wave can be described as follows:
A cdot sin(2pi f t) & 0 leq t < frac{3}{4f} \ 0 & frac{3}{4f} leq t < frac{1}{f} \ end{cases}