Using PWM to generate analog output
As the note says, K should be greater than \$1\$ and an arbitrary value is selected for K. Choosing a different value for K will change \$f_{PWM}\$, the frequency of PWM. \$ie.,\$ if K = 6 then \$f_{PWM}\$ will be 24KHz.
That is a printing mistake. The original equation is,
$$G(dB)_{20KHz} = -10\times log[1+(2\pi fRC)^2] = -14.19\ dB$$