[原创] 十月七日机井 数学部分11道题

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12. What is the total number of different 5-digit numbers that contain all of the digits 2, 3, 4, 7, and 9 and in which none of the odd digits occur next to each other?

偶数只能插在中间，C(3,1)*C(2,1)*C(2,1)=12种，这种题我最糊涂了，不知道这样算对不对？

我是这么算的，三个奇数全排列，然后乘以两个偶数全排列。3x2x2＝12

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16. A three-digit code consists of digits from 0 to 9. If the first digit cannot be 0 or 1, the second digit must be 0 or 1, and the second and third digits cannot be 0 at the same time, how many codes are possible?

C81*C91*2=144

我的算法：C81xC91＋C81xC101=8x9+8x10=152

如果第二位是0，那第三位有9种可能；第二位是1，第三位有10种可能。

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18. What is the remainder when positive integer x is divided by 7

1). X+1 is divisible by 7

2). X+13 is divisible by 7

1) x+1=7a, x=7a-1,余数为1

2）x+13=7a, x=7a-13, 余数为6或1

选A

我觉得这道题应该选D啊，余数都为1

Q. In which quadrants do the roots of the equation lie, given x and y are integers: Y = (x-2)^2 -1

Y = (x-2)^2 -1

Y=x^2+4-4x-1

Y=x^2-4x+3

Y=(x-1)(x-3)

so, either x=y+1  or x=y+3

we can't determine what quadrant exactly x and y lie, so the answer would be E

这道题答案应该是1 2 4象限。不是DS吧？？

这道也不懂，什么叫做方程的解，是和x轴的交点呢还是所有整数解？

如果是整数解，那么...(-1,8),(0,3),(1,0),(2,-1),(3,0),(4,3)...都在第一二四象限啊？？不懂。。。

y=(x-2)^2-1这个方程的解在坐标轴上是一条抛物线，算一下它与x轴，y轴的交点，确定不经过第三象限。