几道数学问题求教

sunniers

请好心人帮我解答以下问题

1. a first-grade teacher use ten flash cards, numbered 1 through 10, to teach her students to order numbers correctly. she has students choose four flash cards randomly, then arrange the cards in ascending order. one day, she removes the cards numbered 2 and 4 from the deck of flash cards. on that day, how many different correct arrangements of four randomly selected cards are possible?

选项A70 B210 C336 D840 E1680

2. if n is a positive integar and n²is divisible by72,then the largest positive integar that must divide n is  

选项 A6 B12 C24 D36 E48

mingzhu99

A,70 吧，就是C8,4 8中选4个，排列按照升序排列。
D,36, 整数和它们平方能被72整除，最大酒食36了

greates

第二题我觉得是48，n^2是72的倍数，72=36*2，说明n至少是6*2=12，那么能整除12的最大正整数是48。

如有不周之处，请指教

mingzhu99

is divisible by 理解错了，应该是48

sunniers

第二题答案是B，12，我就是不明白，请教大家看看，或者说答案错了.

painsper

第一题我做到过，是因为它问的是通过这样的算法，哪个跟9*6一样
27用题目的算法就是27*3与你算到的结果是一样啊

kzang90

72=2^3x3^2
能整除N^2, 所以n^2中至少有2^4x3^2
所以n中至少有2^2x3,所以是12