求解高人。。这题怎么做

Jolie881209 · 发表于 2010-11-3 21:19:18

gwd 1
Q28:
For any positive integer n, the sum of the first n positive integers equals [n(n+1)]/2.What is the sum of all the even integers between 99 and 301?

A.10,100
B.20,200
C.22,650
D.40,200
E.45,150
Answer: b

zhangyunkai5 · 发表于 2010-11-3 21:25:34

是even整数就是偶数哈 100-300
总共101个偶数（400×101）/2就是结果了

BLinging · 发表于 2010-11-3 21:28:27

我算出来怎么是20400呢？

mademperor · 发表于 2010-11-3 21:34:01

我是这样想的.

在99至301的偶数为100,102,...,300, 中点在200
100到110里有5个偶数（包括100自己): 100, 102, 104, 106, 108, 在100-200之间总共有10对这样的数, 所以是50个偶数。于此类推200-300之间也同样有50个偶数。100+298 =400 400*50+200=20220

mademperor · 发表于 2010-11-3 21:34:58

打错了, 应该是100+300 =400 400*50+200=20200

Jolie881209 · 发表于 2010-11-3 21:37:15

题目看不懂啊。。？！求解
Q19:
A total of $60,000 was invested for one year. But of this amount earned simple annual interest at the rate of x percent per year, and the rest earned simple annual interest at the rate of y percent per year. If the total interest earned by the $60,000 for that year was $4,080, what is the value of x?
(1)    x = (3/4) y
(2)   The ratio of the amount that earned interest at the rate of x percent per year to the amount that earned interest at the rate of y percent per year was 3 to 2.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.
Answer:c

Jolie881209 · 发表于 2010-11-3 21:42:19

多谢！！！看懂了

求解高人。。这题怎么做

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