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12₊14₊16₊.........₊40=

Răspuns :

12+14+16+18+20+22+24+26+28+30+32+34+36+38+40=390
12+14+16+...+40=2(6+7+...20)=2*195=390



6+7+8+...+20=1+2+3+...+20-(1+2+3+..5)=20*21:2-5*6:2=210-15=195