Explain why the after-tax cost of equity (common or preferred) does not have to be adjusted by

Question:

Explain why the after-tax (common or preferred) does not have to be adjusted by the marginal income tax rate for the firm?
Cost Of Equity
The cost of equity is the return a company requires to decide if an investment meets capital return requirements. Firms often use it as a capital budgeting threshold for the required rate of return. A firm's cost of equity represents the...