If you have data points, and you want to find a sinusoidal function to fit them, make sure your x-data are in L1 and your y-data are in L2. Then perform the following steps:

1. Press the m button.
2. Press the right arrow key › to CALC.
3. Press the down arrow key ≥ to SinReg: (Sine Regression) and press =.
4. Input your L1 and L2 lists: (where L1 is your x-data and L2 is your y-data) and press = to calculate the parameters of the sine regression formula y = a*sin(b*x + c) + d.
5. Select the softkey for CALCULATE by pressing d or f.

Note: Use the period as an optional argument to improve the model performance if there is an expected or approximate period of the data.