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PUNCTAJ MAXIM+COROANA DACA REZOLVATI CORECT

Intr-o progresie aritmetica (an) gasiti suma primilor :

a) 18 termeni , daca a5+a8+a11+a14=18

b)30 de termeni , daca a11+a14+a17+a20=30

c)2n+8 termeni , daca an +an+3+an+6+an+9=2n+8