[求助]急求PP3几到题

想飞

1. Which of the following inequlities has a solusion set that when graphed on the number line, is a single segment of finite length?
a. x^4>=1
b. x^3<=27
c. x^2>=16
d. 2<=I1I<=5
e. 2<=3x+4<=6
ANS: E
我觉得BCDE都可以呀, 都是有限的.

2.IF r and s are intergers and rs+s is odd, which of the following must be even?
a.r
b.s
c, r+s
d. rs-r
e. r^2+s
ANA: C
RS+S 是偶数, 则RS和S必须都是偶数=> S是偶数或R是偶数或都是ODD. => R+S不一定是偶数呀?

3. IF the operation is defined for all integers a and b by a@b=a+b-ab, which of the following statement must be true for all a, b,c,
ANS: (A@B)@C=A@(B@C)
不明白这是如何得来的?

4.a certain library accesses fines for overdue books as follows. on the first day that book is overdued, the total fine is $0.1, for each additional day that the book is overdue, the total fine is either increased by $0.30 or doubled,whichever results in the lesser amount. What is the total fine for a book on the fourth day it is overdue?
ANS: $0.7
我算了半天都是0.6.

还有两天考了, 那位帮忙回答一下.谢谢!