问题如下：
5. If Skylark implements Lee’s proposal to issue $1 billion of debt and use the proceeds to repurchase common shares, its weighted average cost of capital following the buyback will most likely:
选项：
A.increase to 8.50%.
B.increase to 8.70%.
C.decrease to 8.50%.
解释：
C is correct.
The WACC is the weight of debt times the after-tax cost of debt plus the weight of equity times the cost of equity. Skylark currently has a market value of $8 billion of equity and $2 billion of debt, for an 80/20 ratio. Using Exhibit 1, we find that this capital structure has a 5.5% pretax and 3.52% [5.5 × (1 – 0.36)] after-tax cost of debt and a 10.0% cost of equity. Thus the WACC equals (0.2 × 3.52) + (0.8 × 10.0) = 0.70 + 8.0 = 8.7%. After the buyback, Skylark’s capital structure will be 70/30, which results in a 6.0% pretax cost of debt (3.84% after tax) and a 10.5% cost of equity. Thus, the new WACC will be (0.3 × 3.84) + (0.7 × 10.5) = 1.15 + 7.35 = 8.5%, which is a decrease.
发债后产生的tax shield，使得公司的value提高 [ EBIT (1-T) / r0 ] ，再用value减去debt就是新的equity (老师讲义上面有个例题，李老师也讲了说新的equity不能直接加上发 的debt，因为equity受到的RE会受益于tax shield） 这个新的equity不应该直接用old equity + issued debt 吧？
