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54+52+50+.....+2 urgent

Răspuns :

Scriem suma invers si dam un 2 factor comun:

2+4+6+....+54= 2*(1+2+3+...+27)=2 * 27 * 28/2 = 27* 28 =756

2(27+26+...+1) = 2*27*28/2 = 27*28 = 756