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Calculati:
(1+2+3+4+....+20)


Răspuns :

(1+2+3+4+....+20)=20x21:2=10x21=210
sunt doua metode 1+20+2+19+3+18+4+17+5+16+6+15+7+14+8+13+9+12+10+11= 21+21+21+21+21+21+21+21+21+21= 41+41+41+41+41=82+82+41=164+41=205
si a doua e prin simplificare