[求助]又要問點題目了，麻煩大家幫忙

1 the sum of the first k positive integers is equal to k(k+1)/2,what is the sum of the integers from n to m, inclusive when 0<n<m?

B  m(m+1)/2-n(n+1)/2

C  m(m+1)/2-(n-1)n/2

i chose B,but the key is C

2 DS what is the value of x

  a ([x]-1)(x-2）=0  [x]代表絕對值

  b -1<=x<=1

3  DS 60個animal,2/3 either cow or pig,問多少是cow

  a more than twice as many cows as pigs

  b there are more than 12 pigs

謝謝各位

第一题，注意题目中的inclusive。就是说n也包括在内的。所以应该是c

第二题，由1推出[x]=1或者x＝2，由2只能排除x＝2，x的值还是无法确定，选e

第三题，猪和牛总共40只，假设猪为x只，牛为40－x只

由1得到2x < 40-x =>x<40/3

由2得到x>12

所以x＝13，选c