TL:
1,1+( -2)+3+( -4)+...+19+( -20)
= ( 1-2)+( 3-4)+...+( 19-20)
= -1 -1 -... -1
= -1.10
= -10
2, 1 – 2 + 3 – 4 + . . . + 99 – 100
= ( 1 - 2 ) + ( 3 - 4) + .... + ( 99 - 100 )
= -1 + ( -1) + ....+ ( -1)
= -1.50
= -50
3, 2 – 4 + 6 – 8 + . . . + 48 – 50
= 2 +( – 4 + 6)+( – 8+10) + . . . +( -44+46)+ ( 48 – 50)
= 2+2+2+...+2+( -2)
= 2.12 +( -2 )
= 22
4, -1+3-5+7-...+97-99
= ( -1+3)+( -5+7)+....+( -93+95)+( 97-99)
= -2+( -2)+...+( -2)+2
= -2.24+2
= -46
5, 1+2-3-4+...+97+98-99-100
= ( 1+2-3-4)+...+( 97+98-99-100)
= -4+...+( -4)
= -4.25
= -100
