[讨论][求助]几道资充题求教!

fernandochang

og上的解释看不懂.......麻烦解释一下...谢谢! 255.If x is to be selected at random from set T, what is the probability that 1/4x-5≦0? (1)T is a set of 8 integers. (2)T is contained in the set of integers from 1 to 25, inclusive A):E 253.In the rectangular coordinate system, are the points (r,s) and (u,v)equidistant from the origin? (1)r+s=1 (2)u=1-r and v=1-s. A):C 250.What is the value of a4-b4 (1) a2-b2=16 (2)a+b=8 A):C 245.Is 1/a-b < b-a? (1)ay? (1)x=y+2 (2)x/2=y-1 A):A 243.If the integer n is greater than 1, is n equal to 2? (1)n has exactly two positive factors. (2)The difference of any two distinct positive factors of n is odd. A):B

fengqiwu

1、据题目，x<=20, 1）8个整数的数列，显然不知道数列中的数是否都大于20或小于20或者有些大于20有些小于20，很多种情况，因此不确定，2）包含在1~25的数列中，与1）同，显然也不确定，1）与2）同时考虑也无法判断数列情况，因此E 2、(r,s) and (u,v)两点到原点距离相同，那么有r^2+s^2=u^2+v^2,将条件1）2）代入， 可得上述等式，因此C 3、没看明白 4、1）分别讨论a>1,b>1 ，0b or a