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cat face 300+305+310+315+325+...+390+395=?

Răspuns :

305 = 300  + 5 ·1 
395 = 300 + 95   = 300 + 5 · 19 
suma de 20  termeni 
suma = ( 300  + 395 ) · 20 :2 = 695 · 10 = 6950