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19+20+21+22+...+39 pls am nev repede

Răspuns :

S = 19+20+21+ ... +37+38+39 (avem 39-19+1=21 termeni)

S = 39+38+37+ ... +21+20+19

2S= 58+58+58+...+58+58+58

  S = 58 * 21 : 2 = 609