I don't believe the graphic calculator has a cosine regression
tool, but if you go to STAT, and CALC, there is a sin regression
tool. If you hit enter on that then insert your L values, it will
come up with a sin regression. The sin regression should be the
same as a cosine regression, except that the sin regression should
have a different value of C, usually getting rid of the value of C
altogether will give you the correct regression.