分期免息实际年利率计算

rec = \frac{\frac{price}{n}}{(1+\frac{r}{n})} + \frac{\frac{price}{n}}{(1+\frac{r}{n})^2} + ... + \frac{\frac{price}{n}}{(1+\frac{r}{n})^n}

rec = \frac{\frac{\frac{price}{n}}{(1+\frac{r}{n})} * (1 - (\frac{1}{1 + \frac{r}{n}})^n)}{1- \frac{1}{1 + \frac{r}{n}}} = \frac{price * (1 - (\frac{1}{1 + \frac{r}{n}})^n)}{r}