为什么不能用简化的式子做

NO.PZ2020033002000032问题如下 investment manager who specializes in cret-linkebon is trying to finthe cret-linkeyielspreon a one-yeBB-ratecoupon issuea multinationcompany. With the current market risk-free rate of 2% per annum ana fault rate of 8% for BB-ratebon ana fault loss rate of 70%, a reasonable yielto maturity for this bonis 4.51% B.6.00% 7.50% 8.05% is correct.考点Infer Cret Risk from Corporate BonPrices解析假设收益率为y，则有公式11+y=1∗(1−π)1+rf+f∗π1+rf\frac1{1+y}=\frac{1\ast(1-\mathrm\pi)}{1+r_f}+\frac{\mathrm f\ast\mathrm\pi}{1+r_f}1+y1​=1+rf​1∗(1−π)​+1+rf​f∗π​代入数字，有1/（1+y）=（1-8%）/（1+2%）+（1-70%）*8%/（1+2%）得到 y=8.05% 这道题为什么不能用risk neutral的公式 lam等于sprea以1-RR？

NO.PZ2020033002000032 问题如下 investment manager who specializes in cret-linkebon is trying to finthe cret-linkeyielspreon a one-yeBB-ratecoupon issuea multinationcompany. With the current market risk-free rate of 2% per annum ana fault rate of 8% for BB-ratebon ana fault loss rate of 70%, a reasonable yielto maturity for this bonis 4.51% B.6.00% 7.50% 8.05% is correct.考点Infer Cret Risk from Corporate BonPrices解析假设收益率为y，则有公式11+y=1∗(1−π)1+rf+f∗π1+rf\frac1{1+y}=\frac{1\ast(1-\mathrm\pi)}{1+r_f}+\frac{\mathrm f\ast\mathrm\pi}{1+r_f}1+y1​=1+rf​1∗(1−π)​+1+rf​f∗π​代入数字，有1/（1+y）=（1-8%）/（1+2%）+（1-70%）*8%/（1+2%）得到 y=8.05% 为什么不能用简化方法？简化方法算完等于4.4%ytm-2%=8%*(1-0.7)与精确算法为什么差一倍多了呢？

NO.PZ2020033002000032 问题如下 investment manager who specializes in cret-linkebon is trying to finthe cret-linkeyielspreon a one-yeBB-ratecoupon issuea multinationcompany. With the current market risk-free rate of 2% per annum ana fault rate of 8% for BB-ratebon ana fault loss rate of 70%, a reasonable yielto maturity for this bonis 4.51% B.6.00% 7.50% 8.05% is correct.考点Infer Cret Risk from Corporate BonPrices解析假设收益率为y，则有公式11+y=1∗(1−π)1+rf+f∗π1+rf\frac1{1+y}=\frac{1\ast(1-\mathrm\pi)}{1+r_f}+\frac{\mathrm f\ast\mathrm\pi}{1+r_f}1+y1​=1+rf​1∗(1−π)​+1+rf​f∗π​代入数字，有1/（1+y）=（1-8%）/（1+2%）+（1-70%）*8%/（1+2%）得到 y=8.05% 同样是求ytm rate， 加入我通过 risk neutral的公式= 未来现金流折现求pri两式子相等，带入其他参数比如fault rate， rf rate，loss rate 反求ytm=4.51%请老师指点一下这个并非近似（ytm-rf ≈。。。）的方法，为什么求出来的ytm不对呢。 我这种方法错在哪里呢？ 多谢

NO.PZ2020033002000032 问题如下 investment manager who specializes in cret-linkebon is trying to finthe cret-linkeyielspreon a one-yeBB-ratecoupon issuea multinationcompany. With the current market risk-free rate of 2% per annum ana fault rate of 8% for BB-ratebon ana fault loss rate of 70%, a reasonable yielto maturity for this bonis 4.51% B.6.00% 7.50% 8.05% is correct.考点Infer Cret Risk from Corporate BonPrices解析假设收益率为y，则有公式11+y=1∗(1−π)1+rf+f∗π1+rf\frac1{1+y}=\frac{1\ast(1-\mathrm\pi)}{1+r_f}+\frac{\mathrm f\ast\mathrm\pi}{1+r_f}1+y1​=1+rf​1∗(1−π)​+1+rf​f∗π​代入数字，有1/（1+y）=（1-8%）/（1+2%）+（1-70%）*8%/（1+2%）得到 y=8.05% 题目里面的70%为什么是risk recovery rate,而不是LG