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Aestheticxeri
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14. Calculati următoarele sume:
a) 1+2+3+...+30:
b)2+3+4+...+30;
c)9+10+11+...+30.​

Răspuns :

a) 1+2+3+....+30=

Folosim formula N(N+1):2

In cazul nostru N=30

30(30+1):2

30·31:2

=930:2

=465

8Pandicornul8genial8

a) S=1+2+3+...+30=(30·31):2=456

b) S=2+3+4+...+30=1+2+3+...+30–1=456–1=455

c) S=9+10+11+...+30=1+2+3+...+30–(1+2+3+...+8)=456–(8·9):2=456–36=420