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Sa se determine suma primilor 19 termeni ai unei progresi aritmetice
[tex](an) n \geqslant 1[/tex]
daca
[tex]a1 = - 3[/tex]
si ratia
[tex]r = 2[/tex]



Răspuns :

S19=(a1+a19)•nr de termeni/2
a19=a1+18r=-3+18•2=-3+36=33
S19=(-3+33)•19/2
S19=30•19/2
S19=15•19
S19=285