3 math JJ need explained a bit more and confirm,thanks!!

andrewcan

1. k和p是positive constants，问以下那个方程一定有一个负数解。
I、 X*X+kX+p=0
II、 X*X-kX+p=0
III、 X*X+kX-p=0
选项：（A）I , (B) II, (C) III, (D) II and III (E) I and II and III
（C）

2. In an insurance company, each policy has a paper record and an electric
record. For those policies having incorrect paper record, 60% also having
incorrect electric record; For those policies having incorrect electric
record, 75% also having incorrect paper record. 3% of all policies have both
incorrect paper and incorrect electric records.
If we randomly pick out one policy, what's the probability that the one having
both correct paper and correct electric records? 94%

设incorrect paper record为P, and incorrect electric record is E
60%P=75%E=3%I
then P=1/20I, E=1/25I
so I-PUE=1-(1/20+1/25-3/100)=0.94

3.某类电池以前价格5元，能用1.25小时，现在价格6.9元，能用1.5小时，问价格性能比
增加多少百分比
15%
[em06][em10]

suse

1.用求解的公式(-b+/-根号b^2-4ac)/2a
I 得(-k+/-根号k^2-4p)/2, 不能判断k^2-4p的大小，
II得(k+/-根号k^2+4p)/2, 不能判断k-根号k^2+4p的大小是正还是负
  iii得(-k+/-根号k^2+4p)/2,可以确定，一定有负数解，因为-K是负数，-根号k^2+4p也是负数。

2.对

3.（6.9/1.5-5/1.25）/（5/1.25）=15%

andrewcan

thanks Suse !

about question 1:
根号k^2-4p  should be positive, right?  so( -K - 根号k^2-4p ) / 2 can be negative, right?

Or do we have to consider that K^2-4p is negative, so it has no meaning with 根号, therefore (-k+/-根号k^2-4p)/2 has only one 解, right?

thanks again!

suse

no,因为我们不知道k.p的值，题目是问一定有一个是负数，但I是有可能而已。也有可能没有，所以答案不唯一确定，

对于i的解我们不知道，如果K=0呢？

[此贴子已经被suse于2002-12-10 13:50:42编辑过]