救命啊 SHUXUE

guo573874201

1.If x is a positive integer, is the remainder 0 when 3^x + 1 is divided by 10?
(1)x = 4n + 2, where n is a positive integer.
(2)x > 4答案选A
2. If a code word is defined to be a sequence of different letters chosen from the 10 letters A, B, C, D, E, F, G, H, I, and J, what is the ratio of the number of 5-letter code words to the number of 4-letter code words?


A.5 to 4
B.3 to 2
C.2 to 1
D.5 to 1
E.6 to 1正确

3.  Which of the following fractions has a decimal equivalent that is a terminating decimal?
A 10/189     B 15/196   C16/225   D 25/144   E 39/128    有什么好方法可以看出来呢  


需要指点  谢谢了

anan1982

3.  Which of the following fractions has a decimal equivalent that is a terminating decimal?
A 10/189     B 15/196   C16/225   D 25/144   E 39/128    有什么好方法可以看出来呢

这题我会先化简，然后看分母，如果分母因子 中是2，4，5之类的OK，如果有3之类的我会看看

wyw1018

1、条件1 3^(4n+2)+1=3^2*3^4n+1=9*(3^2)^2n+1=9*9^2n+1=9*(10-1)^2n+1因此只需看9*(-1)^2n+1除10的余数为一个定数，因此充分；条件2 因为3的指数的尾数是3 9 7 1 循环，因此+1有肯被10 整除也可能不被10 整除，不充分
2、5字母排列10×9×8×7×6  4字母排列 10×9×8×7 因此比为6
3、如果分母粒有3 7 11等的指数，而分字又没有这些指数的倍数，必为循环小数。