【费费数学】第六部分（1-5）

tiger1234

well, the problem is that you don't care whether xyz > 0; the problem asks for x(y + z) > 0, since we know that x, y, and z all have the same sign, that means y + z have the same sign as x thus x * (y + z) > 0

[此贴子已经被作者于2004-9-19 9:38:01编辑过]

horsefish

以下是引用tiger1234在2004-9-19 9:37:00的发言：
well, the problem is that you don't care whether xyz > 0; the problem asks for x(y + z) > 0, since we know that x, y, and z all have the same sign, that means y + z have the same sign as x thus x * (y + z) > 0

sorry, 刚刚我把问题写错了.我的问题:由条件（1）可知X、Y 是同号，由条件（2）可知Y、Z 也是同号，则X、Y、Z 都是同号

我不能肯定是否得到这个结论:X、Y、Z 都是同号

[此贴子已经被作者于2004-9-19 11:07:12编辑过]

tiger1234

well if xy > 0 and yz > 0 => xy * yz > 0 = x * y^2 * z > 0 since y^2 > 0 => x*z > 0 thus x and z have the same sign as well, since x and y have the same sign, y and z have the same sign, and x and z have the same sign, thus x, y, z all have the same sign

horsefish

以下是引用tiger1234在2004-9-19 11:09:00的发言：
well if xy > 0 and yz > 0 => xy * yz > 0 = x * y^2 * z > 0 since y^2 > 0 => x*z > 0 thus x and z have the same sign as well, since x and y have the same sign, y and z have the same sign, and x and z have the same sign, thus x, y, z all have the same sign

说服我了. 非常谢谢! 将来有机会姐姐请你吃饭表示感谢!

tiger1234

YAY!!!

bibilioteke

以下是引用耳朵在2004-2-10 4:07:00的发言：
第二题，费费详解里面的答案是B，应该是错误的。正确答案应该是D。
因为可以从展开后的等式得出：A=2C并且B=C^2，那么第一条B=0，则C=0，因此A＝0是可以推出的。因此两个statement都独立有效，选D。

贊成

xyl927

3. A

(1) N =3

C(2, N) / C(2,10) = 1/15

(2) N has so solution.

C(1,2)P(1-P) = 7/15

or C(1,2)C(1, N)C(1, 1-N) / C(2,10) = 7/15

[此贴子已经被作者于2004-11-9 1:23:01编辑过]

margaret1013

以下是引用xyl927在2004-11-9 1:22:00的发言：

3. A

(1) N =3

C(2, N) / C(2,10) = 1/15

(2) N has so solution.

C(1,2)P(1-P) = 7/15

or C(1,2)C(1, N)C(1, 1-N) / C(2,10) = 7/15

(2)用不着排列，只要C(1, N)C(1, 1-N) / C(2,10) = 7/15

1284902

第一题：我认为答案是91%。60% of incorrect paper record(P')=75% of incorrect electric record(E')=3% of all, 即0.6P'=0.75E'=0.03=P'E'，求出P'=0.05，E'=0.04，所以

所以P(correct paper )=1-P'=1-0.05=0.95

E( correct electric record)=1-E'=1-0.04=0.96

所以PE(both correct paper and correct electric record)=P+E-1=0.91

0.94的答案是指correct paper or correct electric paper,而不是PE(both correct paper and correct electric record)=1-P'-E'+P'E'=0.94

QQ虫

5、1/a+1/b+1/c+1/d+1/e=1，a…e全是不同的正整数，问：a+b+c+d+e的least possible value？=3+4+5+6+20

好像还是没有人说怎么解出这个答案的????