Question

920=80/(1+r)+80/(1+r)^2+1000/(1+r)^ 答案r=12.78% 结果是怎么算出来的

Answer 1

920=80/(1+r)+80/(1+r)^2+1000/(1+r)^2

==> 920 / 80 = [(1+r) + 1 + 12.5] / (1+r)^2

==> 11.5 (1+r)^2 = 14.5+r

==> 11.5r^2 + 22r -3 = 0

==> (r+22/23)^2 = 3/11.5 + (22/23)^2 = (√622 / 23)^2

==> r = ±√622/23 - 22/23

r= 0.1278 或 -2.04

至于为什么要舍掉一个负数值,可能你的问题没有列所有的条件,请自己判断.