Then you want to add the present value of the principal and that's the formula.

最后还要加上债券面值的现值,这就是债券定价公式

耶鲁公开课 - 金融市场课程节选

Your mortgage payment is constant through time but the fraction of it that goes to paying off principal versus interest changes through time and it just changes as dictated by this formula.

抵押贷款每月还款是固定的,但是其中本金和利息的比例关系,会随时间而改变,这是由这个公式所决定的

耶鲁公开课 - 金融市场课程节选