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calucati : (3+6+9... 369) - (2+4+6+...246)

Răspuns :

(3+6+9+... +369) - (2+4+6+...+246) =

= 3×(1+2+3+...+123) - 2×(1+2+3+...+123) =

= 1+2+3+...+123 =

= 123 × 124 : 2 =

= 7 626