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Calculati suma primilor 10 termeni intr .o progresie aritmetica daca a1=5 .r=8

Răspuns :

a1=5 
r=8
Sn=n(a1+an)/2
a10=a1+(n-1)*r=5+9*5=50
S10=10(5+50)/2=10*55/2=5*55=275