1-2+3-4+``````+33-100=____.

原式 = (1-2) + (3-4) + (5-6) + ... + (99-100)
= -1 * (100 / 2)
= -50

等差数列。把三项并在一起，就可以发觉它变成了
0+3+6+……+96+100=（0+96）*33/2+100=1684

-1*50 =-55