大一經濟學原理相關問題

以下三題是我在考大一經濟學原理的時候考的問題跟折現值還有利率有關有點搞不太懂折現值意思是什麼老師上課講很快我都用這個公式(老師上課筆記中) 1.A(1 i)=R 2.A=R/1 i 不過我想問的是A.R哪個是主要在算折現值? 是本金A嗎??有時候都會被搞混會搞不清楚題目到底是要問A 還是R請高人指點下面題目的中文意思為什麼會選這選項?1.The present value of $100 to be received in a year is A)less than $100 and falls as the interest and rises.B)less than $100 and rises as the interest and rises.C)more than $100 and falls as the interest and rises.D)less than $100 and rises as the interest and rises.Ans:A自己的想法是想是說折現值100元然後要問本來投資多少的意思嗎??有點看不太懂