PP-T1-DS-144

[lucyyeh]

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 I =1000〔(1 +r/100)^n - 1〕 , 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
可是我選D
我算2的 -5<r<3 (可是利率沒有算負的吧??)
不是可以回答題目的interest rate有沒有大於8%嗎？

請問我是那邊算錯了呢？

[pimi]

我也是選D
我是這樣算的
(1+r/100)^2>1.15
1+2r/100+r^2/10000>1.15
2r/100+r^2/10000>0.15
200r+r^2/10000>0.15
r(200+r)>1500
這樣看來r是絕對要大於8%
請問大大我是哪裡算錯呢？？

[小花]

8% 兩年 帶進去算I =1000〔(1 +r/100)^n - 1〕
求得166.4
(1) The deposit earns a total of $210 in interest in the first two years.
是的
(2) ( 1 + r/100 )^2 > 1.15
可能小於 也可能大於

[pimi]

不太了解為何
(2) ( 1 + r/100 )^2 > 1.15
可能小於 也可能大於

請小花大大可否再解釋呢？？

[puja]

不太了解為何
(2) ( 1 + r/100 )^2 > 1.15
可能小於 也可能大於

請小花大大可否再解釋呢？？

用公式拆解:

(1+r/100)^2 > 1.15
=1+r/100+(r/100)^2 >1.15
=(r+100)^2 > 11500
=r+100 > 105(大約值)
所以 r 可能大於也可能小於 8