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Aratati ca 9×(5+10+15+...+200):41 este patrat perfect, please.

Răspuns :

9×[5×(1+2+3+...+40)]:41
9×[5×(40×41:2)]:41
9×[5×(20×41)]:41
9×(5×820):41
9×4100:41
36900:41
900=30²