prep 1 DS-144

144.  14635-!-item-!-187;#058&010061

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 , 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 $210 in interest in the first two years.

(2) (1 + r / 100) ^2  > 1.15

ANS : A

未啥B 不行??

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直接解(1 + r / 100) ^2  > 1.15

r% >（更号下1.15）-1

(r%)^2 > 1.15+1-2*(更号下1.15）>0.25（这里 更号下1.15 > 1）

r%>0.5

r>0.005

所以不能充分说明问题...

我想应该这样做吧？