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calculati 63+66+69+...537

Răspuns :

63+66+69+339=537

coroana? :DDD
3(21+22+23+...+179)=
3(1+2+3+...+179-1-2-3-...-20)
3(179*180/2-20*21/2)
3(179*90-210)=
3(16110-210)=
3*15900=47700