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Calculeaza: (11+22+33+55+......... +99)

Răspuns :

11+22+33+44+55+...+99 =

= 11×(1+2+3+4+5+...+9) =

= 11 × 9 × (9+1) : 2 =

= 99× 10 : 2 =

= 99 × 5 =

= 495