老师，数学基础课这题 在可重复使用letter时，为什么在two letter code时不用✖️A[2,2]，在three letter code时 不用✖️A[3,3]呢？ 我是知道 当需要“选，拿，取”时，我们要用C，；但是我对具体什么时候需要用到A 还是有些模糊。能不能麻烦老师您解释一下我们什么时候会用到A？特别是在我们已经用到C后，什么时候需要在C基础上 再✖️A（e.g. C[3,26]✖️A[3,3]）？ 谢谢！

In the country of Veltria, the past two years' broad economic recession has included a business downturn in the clothing trade, where sales are down by about 7 percent as compared to two years ago. Clothing wholesalers have found, however, that the proportion of credit extended to retailers that was paid off on time fell sharply in the first year of the recession but returned to its prerecession level in the second year. Which of the following, if true, most helps to explain the change between the first and the second year of the recession in the proportion of credit not paid off on time? (A) The total amount of credit extended to retailers by clothing wholesalers increased between the first year of the recession and the second year. (B) Between the first and second years of the recession, clothing retailers in Veltria saw many of their costs, rent and utilities in particular, increase. (C) Of the considerable number of clothing retailers in Veltria who were having financial difficulties before the start of the recession, virtually all were forced to go out of business during its first year. (D) Clothing retailers in Veltria attempted to stimulate sales in the second year of the recession by discounting merchandise. (E) Relatively recession-proof segments of the clothing trade, such as work clothes, did not suffer any decrease in sales during the first year of the recession. 这道题错选了A。 思路如下： 题目问的是为什么在经济不好，衣服卖不动的情况下，第一年零售商还不上钱第二年就能还上了。讲的是零售商还钱的问题。 当时留下的是A,C,D 排除D是因为觉得，即使刺激销售量衣服打折也不能说明能还上钱。可能打折卖1块，纵使家当全卖了也不够。 排除C是因为觉得， 那些本身经济状况就很差的在第一年被淘汰掉以后，剩下的那些没说经济状况如何啊，如果也不咋地，第二年还是得淘汰啊。 A选项当时觉得，即使credit extended，对pay off on time也无太大用处，但最起码跟原文意思接近。所以选了A。

请教老师这题是否选择D

此题不会

这道题请老师讲一下是否选择C

请教老师此题，是否答案为（2M-0.4Mπ）/2

求解

有个洗车行，洗一辆车是 8 块，一个天能挣 200 元。然后打算加价，但是每增加 0.5块，就会减少 10 个顾客，问公式表达收入。答案有 200+(p-8)*0.5/10; 200+(p- 8)*10/0.5; 200-(p-8)*0.5/10 之类的。 感觉上面的答案不对

机经课上没有讲的几何

请问

请教老师