数学JJ 256-303 整理中．．．

yangliu0401

以下是引用wangxiaoqing在2009-3-21 7:35:00的发言：

294. DS有一个set里面全是n的倍数，n是大于1的整数（举例说比如-2n,-n,0,n,3n等都在里面），问N是多少
   

（1）-7在里面（2）49在里面

正确答案是D
   

a吧，2 的话49和7可以是n

1 is right

2: 49=2n wrong, 49=-2n, wrong, 49=0, wrong, 49=+/-3n wrong because n is an integer

49=-n wrong because n <0, so only n =49, so n can be determined, so 2 is fine

so answer is D

题目不是说set里全是n的倍数么，然后举例说比如-2n,-n,0,n,3n等都在里面，那条件2是不充分的呀：

条件2说明 n=49 or 7. 当n=7时，49就是7n，这也符合题目中描述的"set里全是n的倍数"这个前提呀。

所以答案还是A吧

[此贴子已经被作者于2009-3-23 23:18:07编辑过]

yangliu0401

298. a,b>1 問a=?

1) a^b = 9

2) (a^b-1) (a^b+1) = 64

JJ作者：=>  D

请问，条件2怎么能充分呢？

请问，条件2怎么能充分呢？

yangliu0401

303. S0 = 0, 问S100=？   1. Si - 1 比Si 小i。  2. Si-1 ^ 2 - Si ^ 2 = 1. 

JJ作者：我记得选得是D。
                

Wangxiaoqing 1 is fine; 2: can get positive or negative S    so answer should be A
            

有N人帮忙讨论一下303嘛

有N人帮忙讨论一下303嘛

wangxiaoqing

259  (ds)3^k除以10的余数
  
   

1）k是8的倍数   1倍于4  2倍余8  3倍余6……
   

2）k是12的倍数    JJ作者：选D

能确定吗？

The answer is D, the remainder for both is 1

wangxiaoqing

298. a,b>1 問a=?

1) a^b = 9

2) (a^b-1) (a^b+1) = 64

JJ作者：=>  D

请问，条件2怎么能充分呢？

请问，条件2怎么能充分呢？

(1): 3^2, sufficient

(2): a^2b=64; could be 2^6,4^3,8^2, so a could be 2,4,8, so not sufficient

so answer is A if a,b are integers >1

wangxiaoqing

题目不是说set里全是n的倍数么，然后举例说比如-2n,-n,0,n,3n等都在里面，那条件2是不充分的呀：

条件2说明 n=49 or 7. 当n=7时，49就是7n，这也符合题目中描述的"set里全是n的倍数"这个前提呀。

所以答案还是A吧

so there is 7n in the set?

2: If this is the case, 49=7n; or n=49 and 7n=7*49

1: 7n=7 n=1, not >1, so n must be seven

is this what you mean?

wangxiaoqing

278.(DS)问的是a>1吗？根据条件1求得a>1或者a＜-1,根据条件2求得a>0，
   

C为什么不？

行

(1)+(2)

a>0, a>1, a<-1

from a>0, a>1, we get a>1

from a<-1 we get a<-1

from a>1,a<-1, both must be satisfied; so a>1 or a<-1

so a could be positive or negative, so inconclusive

so E

wangxiaoqing

208.一个直角三角形给了斜边的长度和一个内角为30度,求斜边上的高,斜边长度是40
ＪＪ作者 答案是10

答案应该是10根号3吧

40*sin30*cos30=40*1/2*根号3/2=10根号3

The height of the 斜边 divides it into two parts, one with the longer length and one with the shorter length. Our goal is to find the shorter length. The hypotenuse for the shorter length is 20. The angle facing the shorter length is 30 degree. So the shorter length is ten. So the answer is ten.

sunshuai856

以下是引用wangxiaoqing在2009-3-24 3:39:00的发言：

278.(DS)问的是a>1吗？根据条件1求得a>1或者a＜-1,根据条件2求得a>0，
   

C为什么不？

行

(1)+(2)

a>0, a>1, a<-1

from a>0, a>1, we get a>1

from a<-1 we get a<-1

from a>1,a<-1, both must be satisfied; so a>1 or a<-1

so a could be positive or negative, so inconclusive

so E

可是C的意思是两个同时满足啊？

初中的时候解方程经常是这样的啊，求交集

[此贴子已经被作者于2009-3-24 19:32:57编辑过]

wangxiaoqing

sunshuai856, trust me, the answer is E for sure. It is a>1 or a<1. a>1 is used with a>0 to find their intersection, not affect a<-1 because both (1) and (2) must be satisfied.

Hope this helps