(1)1-2+3-4+5-6+.-100+101
=(1-2)+(3-4)+(5-6)+.+(99-100)+101
=(-1)+(-1)+(-1)+.+(-1)+101
=-50+101
=51
(2)1+2-3-4+5+6-7-8+.+21+22-23-24
=(1+2-3-4)+(5+6-7-8)+.+(21+22-23-24)
=(-4)+(-4)+.+(-4)
=(-4)*6
=-24