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calculati suma primilor 20 de termeni ai unei progresii aritmetice in care aunu=4 si r=6

Răspuns :

a1=4
r=6
a20=a1+19r=4+19·6
a20=118
S20=20(a1+a20):2
S20=10(4+118)
S20=10·122
S20=1220