求助呀~~~GWD1套 数学10,28,29

summerwaker

Q10:
Aset of numbers has the property that for any number t in the set, t + 2 is inthe set.  If –1 is in the set, which ofthe following must also be in the set?

I.                    -3
II.                  1
III.                 5


A.    Ionly
B.    IIonly
C.    Iand II only
D.    IIand III only
E.    I,II, and III


答案是D，为什么-5不可以呢?t+2=-1的时候，t=-3,也是在set 里面。
Q28:
Forany positive integer n, the sum of the first n positive integers equals[n(n+1)]/2.  What is the sum of all theeven integers between 99 and 301?

A.     10,100
B.     20,200
C.     22,650
D.    40,200
E.     45,150
100,102,104....298,300。 N=100，A1=100.An=300.用求和公式Sn=(A1+An)*n/2=20000
为什么答案是B呢？
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?Q29:
June25, 1982, fell on a Friday.  On which dayof the week did June 25, 1987, fall? (Note: 1984 was a leap year.)

A.     Sunday
B.     Monday
C.     Tuesday
D.    Wednesday
E.     Thursday
完全没有clue啊~~~~


thx~求助

luluicecream

是t=-1哦， 不是t+2=-1.

只能往上面推，t=-1, t+2=1

t=1, t+2=3,

t=3, t+2=5.

根据条件不可以推出-3的。

luluicecream

"sum of all the even numbers"

"the first n positive integers equals[n(n+1)]/2."

The formula given is not for "sum of all EVEN numbers", it's for "ALL" first n positive integers.

luluicecream

1982~ 1987, 5 years, 1984 is leap year.

5 * 365 +1 =1862

1862/7 (7 days/week)  Reminder:6.

Friday+6 days = Sat., Sun., Mon., Tue., Wed., Thur.


Answer= Thursday.

summerwaker

1982~ 1987, 5 years, 1984 is leap year.

5 * 365 +1 =1862

1862/7 (7 days/week)  Reminder:6.

Friday+6 days = Sat., Sun., Mon., Tue., Wed., Thur.


Answer= Thursday.

-- by 会员 luluicecream (2011/11/23 21:41:50)

谢谢~