很多不会....ds, prep1-144;prep1-147,拜托了

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 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

147.       14800-!-item-!-187;#058&010134

Each employee on a certain task force is either a manager or a director.  What percent of the employees on the task force are directors?

(1) The average (arithmetic mean) salary of the managers on the task force is $5,000 less than the average salary of all employees on the task force.

(2) The average (arithmetic mean) salary of the directors on the task force is $15,000 greater than the average salary of all employees on the task force.

144答案是A，147答案是C, 不明白呀，不明白，大家帮我看看，是怎么做出来的？

147

设x个manager，平均工资a，y个director，平均工资b，

由1推出，(ax+by)/(x+y)-5000=a

由2推出，(ax+by)/(x+y)+15000=b

第一个等式乘3＋第二个等式推出4ax＋4by＝3ax＋3ay＋bx＋by

两边同时除以y推出（a－b）x/y＝3a－3b

所以x/y等于3

143题不会，感觉选A，

等牛人解答，

144题答案肯定是 A

为什么从条件2)我们推不出来r是否大于或小于百分之8呢?

只需要这么考虑就可以了, 把0.08代到公式(1+0.08)^2 =1.166, 显然大与1.15

着也就是说如果r比8%小一点点, 我们完全可能让(1+r/100)^2>1.15.

当然如果r>0.08, 我们更没问题让(1+r/100)^2>1.15

所以条件2不能给出明确答案r是不是大于8%.

zhe

刚才double check了一下，144果然选A

我明白了，拜谢楼上各位热心的LS~~~~~~~~~

haha, i am so silly. thanks