r"""
        :math:Add formula

def dre_dtau(self, pars):
        r"""
        :math:Add formula
        """
        self._set_parameters(pars)
        # term 1
        num1 = self.c * self.w * self.otc1 * np.cos(self.ang)
        term1 = num1/self.denom

        # term 2
        num2a = self.otc * np.cos(self.ang)
        num2b = 1 + num2a
        denom2 = self.denom ** 2
        term2 = num2b / denom2

        # term 3
        term3 = 2 * self.c * self.w * self.otc1 * np.cos(self.ang) + self.otc2

        result = self.sigmai * self.m * (term1 + term2 * term3)

        return result