1/
1+( -2)+3+( -4)+. . . +19+(-20)
=[1+(-2)]+[3+(-4)]+...+[19+(-20)] (có 10 cặp)
=(-1) + (-1) +...+(-1)
=-1.10
=-10
2/
1-2+3-4+....+99-100
=(1-2)+(3-4)+....+(99-100) (có 50 cặp)
=(-1) + (-1) +....+ (-1)
=-1.50
=-50
3/
2-4+6-8+. . . . . +48-50
=(2-4)+(6-8)+. . . . . +(48-50) (có 25 cặp)
= (-2) + (-2) +...........+ (-2)
=-2.25
=-25
4/
-1+3-5+7- . . . . +97-99
=(-1-99)+(-2-97)+....+(-49-51) ( có 50 cặp)
=(-100) +(-100) +....+(-100)
=-100.50
=-5000
5/
1-2+3-4+. . . . . +97-98+99-100
=(1-2+3-4)+. . . . . +(97-98+99-100) ( có 25 cặp)
= (-2) +...........+ (-2)
=-2.25
=-50