20. If 。represents one of the operations +, –, and ×, is k。（x+m）=(k。x)+(k。m) for all numbersand k, m, x?

(1) k·l is not equal to l·k for some numbers k.

(2) 。represents subtraction.

answer:d

donot understand

25. A total of 774 doctorates in mathematics were granted to United States citizens by American universities in the 1972-1973 school year, and W of these doctorates were granted to women. The total of such doctorates in the 1986-1987 school year was 362, and w of these were granted to women. If the number of doctorates in mathematics granted to female citizens of the United States by American universities decreased from the 1972-1973 school year to the 1986-1987 school year, was the decrease less than 10 percent?

(1)1/10<W/774<1/9

(2)W=w+2

answer is C 没思路， 另外我想贴图片，为什么老贴不上呢？

20. If 。represents one of the operations +, –, and ×, is k。（x+m）=(k。x)+(k。m) for all numbers k, m, x?

(1) k·l is not equal to l·k for some numbers k.

(2) 。represents subtraction.

问如果。是加,减,乘中的一个, 那对所有的k, m, x来说, k。（x+m）=(k。x)+(k。m)是否成立?

1) 如果k。l不等于l。k, 那么说。就不是加或乘, 所以用排除法就知道。是减, 那么我们知道k -(x + m) 并不等于k - x + k - m所以从1)我们可以知道答案

2) 直接告诉我们说。是减, 那么如1)代入可知, 所以2)自己也可以知道答案

所以是丁(甲乙丙丁), 这外国人要能看懂我就真的服了他了...

20. If 。represents one of the operations +, –, and ×, is k。（x+m）=(k。x)+(k。m) for all numbersand k, m, x?

(1) k·l is not equal to l·k for some numbers k.

(2) 。represents subtraction.

answer:d

donot understand

句号"."代表+,-,*(加或减或乘), 问是不是所有k,m,x对k。（x+m）=(k。x)+(k。m) 都成立

条件1如果句号"."为加法或乘法则K.L恒等于L.K

即只有句号"."为减法是有时候成立,有时候不成立>>>K.（x+m）=(k。x)+(k。m)等式不成立

(把. 换成减号, 式子不成立)

条件2直接告诉为减法了

25. A total of 774 doctorates in mathematics were granted to United States citizens by American universities in the 1972-1973 school year, and W of these doctorates were granted to women. The total of such doctorates in the 1986-1987 school year was 362, and w of these were granted to women. If the number of doctorates in mathematics granted to female citizens of the United States by American universities decreased from the 1972-1973 school year to the 1986-1987 school year, was the decrease less than 10 percent?

(1)1/10<W/774<1/9

(2)W=w+2

answer is C 没思路， 另外我想贴图片，为什么老贴不上呢？

求(w-W)/W<1/10?

由条件2 >>>(W-w)/W= 2/W

再加入条件1 : 1/10<W/774<1/9>>> 774/10<W<774/9 >>> 2/77.4 <2/W=<2/86 >>> 2/77.4<(W-w)/W< 2/86