求助OG上的数学

Cassie88

请教解释下OG 12上数学PS 第87 and 98  and 157题的解释。。

OG上的解释没看明白！

87. A necklace is made by stringing N individual beads together in the repeating pattern red bead, green bead, white bead, blue bead, and yellow bead. If the necklace design begins with a red bead and ends with a white bead, the N could equal

A.16
B. 32
C. 41
D. 54
E. 68


98. On a scale that measures the intensity of a certain phenomenon, a reading of n+1 corresponds to an intensity that is 10 times the intensity corresponding to a reading of n. On that scale, the intensity corresponding to a reading of 8 is how many times as great as the intensity corresponding to a reading od 3?

A. 5
B. 50
C. 10^5
D. 5^10
E. 8^10-3^10



157. For any positive integer n, the sum of the first n positive integer 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

qfxl

87.试5个选项中谁是5n+3这种形式就行了。

qfxl

98.这可以看做是一个a*10^n的等比数列。答案是a*10^8除以a*10^3选C。

qfxl

157.不必被题干中的公式干扰，直接用等差数列求和公式Sn=nA1+[n(n-1)d]/2就可以了。n是数列的项数，A1是第一项，d是公差。对于题目来说第一项是100，公差是2，项数是101，计算的20200，选B。

Cassie88

为什么是5n+3呢。。。
就是这里不是很明白，希望牛人能详细解释下！