求教两道数学题~大侠们帮帮忙

杞子纸萤

第一题：A store purchased 20 coats that each cost an equal amount and then sold each of the 20 coats at an equal price. What was the store's gross profit on the 20 coats?
If the selling price per coat had been twice as much, the store's gross profit on the 20 coats would have been $2400
(2)If the selling price per coat had been $2 more, the store's gross profit on the 20 coats would have been $440
答案是B  不懂原题的意思。。。
第二题 If n is a positive integer and r is the remainder when n^2-1 is divided by 8, what is the value of r?
(1)  n is odd.
(2)  n is not divisible by 8
答案是A
但是我计算出来0,3,7 都是n^2-1可能的余数呀 为什么只需要A就能计算出r?
求教。。。

杞子纸萤

没人来么？给自己顶一下。。。

猪王山

读题目要求。。。value就是只有唯一确定的值

出国小土鳖

第二题 If n is a positive integer and r is the remainder when n^2-1 is divided by 8, what is the value of r?
(1)  n is odd.
(2)  n is not divisible by 8


n^2-1/8的余数可能是0 3 7  而奇数的平方-1肯定是一个偶数，偶数除以8的余数肯定也是偶数 所以就是0喽

然后我用麻烦的方法给你算一遍
设任意奇数n=2x-1   n^2-1=(2x-1)^2-1=4X^2-4x=4*x*(x-1)  要么x是2的倍数 要么x-1是2的倍数 所以就被8整除了

Heathrow

意思就是一家店以同样的价格买了20条，又以一个价格卖出，求总共利润。
设买价为x，卖价为y.
(1)  (2y-x)*20=2400
(2)  （y+2-x）*20=440
第二个可以算出y-x，这样就能算出总利润了。

杞子纸萤

啊哈~终于弄明白了 太谢谢你们啦~