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Sa se găsească suma primilor 100 termeni ai unei progresii aritmetice daca a1=2,a5=14

Răspuns :

a1=2
a5=14
a5=a1+4r
2+4r=14
4r=12
r=3
S100=(a1+a100)x100:2
a100=(a1+a1+99r)×50
S100=(2+2+99r)×50
S100=(4+99r)×50
S100=(4+99×3)×50
S100=(4+297)×50
S100=331×50
S100=16550