数学讨论稿306 PREP原题考古 第二个条件肿么算捏。。。

以梦为马

If $1,000 is deposited in a certain bank account and remains in the account along with any accumulated interest, the dollar amount of interest, I=1000[(1+r/100)^2-1] , earned by the deposit in the first n years is given by the formula  , where r percent is the annual interest rate paid by the bank.  Is the annual interest rate paid by the bank greater than 8 percent?

[1]The deposit earns a total of $210in interest in the first two years.

[2](1+r/100)^2>1.15

A

huxleyxu

(1+r)2>1.15   如果1+r的平方等于1.15   那么1.15<（1+0.8）2     r就小于0.8
但是(1+r)2得值没有上线，所以1+r可以无限大，这么说来，r就有可能小于0.8也可能大于0.8
不充分，我觉得是

roundroundyy

我刚做了这题，也觉得b不对，做了记号，还没来得及问