Explicație pas cu pas:
Utilizăm formula: Sn=a₁×n+r×n×(n-1)/2
S₁₀=(a₁×10+r×10×9)/2=10a₁+45r
10a₁+45r=100
2a₁+9r=20 2a₁=20-9r
S₃₀=(a₁×30+r×30×29)/2=30a₁+435r=900
2a₁+29r=60 2a₁=60 -29r
20-9r=60-29r
20r=40
r=2
a₁=(20-9×2)/2=1
S₅₀=(a₁+a₅₀)×50/2=[(1+(1+49×2)]×50/2=(1+99)×50/2=5000/2=2500
==lucianglont666==
Mult succes!
