即狗#73题答案有误后又发现#79也有问题。。。

karateguy

73.   整数小于500 求被7除余1 被3除余2   这样的数有几个
记得有24 .21.72

此题答案应该是23吧，狗狗给的答案是24.。。

我的算法：
设通项X=Am+B
X=7a+1=3b+2   则X最小时当a=1, b=2时为8，7与3的最小公倍数为21
则通项为X=21m+8<500
则m=23


79.DD一个数[x]表示不小于X的最近的整数，请问[2x]=0？
1）[x]=0
2) [3x]=0
当时最后一题了，没来的细想，选了B，不知道对不对

选B

1)不就是说x<=0, 2) 说3x<=0吗，，，那x有可能是0的呀，所以应该E吧！！！！

大家来讨论讨论呀！！

jtrchina

The way you think to get the solution is perfectly right,but you should have got the answer "24" if you had taken "m=0" into account.
21m+8<500--> m<23.42.So m should be some value from 0 to 23,inclusive.
There are 24 possible values as the original issue says.

karateguy

楼上。。。。谢谢。。。。牛人出现。。。

jtrchina

#79:
B should be the best answer:
The flaw in premise(1) is so obvious that we can just assign 2 real numbers,-0.5 and -0.4,with one breaking and the other defending the result:[2x]=0
Let's move on to the second:
It tells us that -1<3x<=0 -->-2/3<2x<=0
So no matter x equals to zero or not,the mere result is [2x]=0
That's why B,I think.

karateguy

啊呀，原来问的是『2x』...谢～～～～～～～～～～～LS...