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Calculati:
18+20+22+.................+30



Răspuns :

18+20+22+ ...+30=
2·9+2·10+2·11+...+2·15=
2·(9+10+11+ ...+15)=
15·16:2=
15·16-(0+2+4+...+16)=
240-(2·0+2·1+2·2+...+2·8)=
240-2·(0+1+2+..+8)=
240-8·9:2
240-8·9=
240- 72=
168