The equations for Net Income (NI, also known as Earnings or Net Profit After Tax) and Cash Flow From Assets (CFFA, also known as Free Cash Flow to the Firm) per year are:

###NI=(Rev-COGS-FC-Depr-IntExp).(1-t_c)###

###CFFA=NI+Depr-CapEx - \varDelta NWC+IntExp###

For a firm with debt, what is the formula for the present value of interest tax shields if the tax shields occur in perpetuity?

You may assume:

- the value of debt (D) is constant through time,
- The cost of debt and the yield on debt are equal and given by ##r_D##.
- the appropriate rate to discount interest tax shields is ##r_D##.
- ##\text{IntExp}=D.r_D##