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S= 1+3+5+7+...+19 

Repede va roooog


Răspuns :

(1+2+3+....+19)-(2+4+6+...+18)= 19×20:2 - 2×9×10:2= 19×10 - 9×10 = 10×(19-9) = 10×10 = 100