A quantity that repeats in a steady cycle can be modeled by y = a sin(b(x - c)) + d. The amplitude a is half the distance from low to high, the midline d is their average, the period is 2π ÷ b, and c slides the wave left or right. Read the three numbers from a table or a graph, write the model, and check it between the readings. Adding to the output shifts the wave up, multiplying stretches it, and changing the input shifts or squeezes it sideways.