[tex]\it a)\ \ Avem\ 1004 \ termeni\ pozitivi \ \d s i\ 1 004\ termeni\ negativi,\ pe\ care\ \hat\imath i\ vom\ grupa:\\ \\ (7-5)+(8-7)+(11-9)+\ ...\ +(2013-2011)=2\cdot1004=2008[/tex]
b)
[tex]\it 11+12+13+\ ...\ +78=1+2+3+\ ...\ +10 +11+12+13+\ ...\ +78-\\ \\ -(1+2+3+\ ...\ +10)=\dfrac{78\cdot79}{2}-\dfrac{10\cdot11}{2}=39\cdot79-55=3081-55=3026[/tex]
c)
[tex]\it 5+10+15+\ ...\ +95=5(1+2+3+\ ...\ +19)=5\cdot\dfrac{19\cdot20}{2}=950[/tex]
[tex]\it d)\ 5+8+11+\ ...\ +305=\dfrac{(5+305)\cdot101}{2}=155\cdot101=15655[/tex]
