Răspuns :
c=1-2+3-4+5-6+.....+19-20=
=(1-2)+(3-4)+(5-6)+.....+(19-20)= avem 10 paranteze
=-1-1-1+....-1=-10
=(1-2)+(3-4)+(5-6)+.....+(19-20)= avem 10 paranteze
=-1-1-1+....-1=-10